Search for binary central stars of the SMC PNeTable 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/

Search for binary central stars of the SMC PNethanks: Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/

M. Hajduk Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland    M. Gładkowski Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland Centrum Astronomii UMK, ul. Gagarina 11, 87-100 Toruń, Poland    and I. Soszyński Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
Received ; accepted
Key Words.:
ISM: planetary nebulae: general – stars: binaries: general – stars: binaries: symbiotic – galaxies: Magellanic Clouds
Abstract

Context:

Aims:The Optical Gravitational Lensing Experiment (OGLE), originally designed to search for microlensing events, provides a rich and uniform data set suitable for studying the variability of certain types of objects. We used the OGLE data to study the photometry of central stars of planetary nebulae (PNe) in the Small Magellanic Cloud (SMC). In particular, we searched for close binary central stars with the aim to constrain the binary fraction and period distribution in the SMC. We also searched for PNe mimics and removed them from the PNe sample.

Methods:We identified 52 counterparts of PNe in the SMC in the I-band images from the OGLE-II and OGLE-III surveys. We analysed the time-series photometry of the PNe. Spectra of the photometric variables were obtained to constrain the nature of the objects or search for additional evidence for binarity.

Results:Eight variables were found. Of these, seven objects are PNe mimics, including one symbiotic star candidate. One close binary central star of PN with a period of 1.15 or 2.31 day was discovered. The obtained binary fraction for the SMC PNe and the observational biases are discussed in terms of the OGLE observations.

Conclusions:

1 Introduction

Large-scale photometric surveys provide an excellent opportunity to improve our understanding of the nature of the planetary nebula (PN) phenomenon. Using large photometric surveys allowed researchers to monitor the brightness of hundreds of central stars of planetary nebulae (CSPNe, Miszalski et al. 2009a; Lutz et al. 2010; Jones et al. 2011; Hajduk et al. 2011). The number of known binary CSPNe was more than doubled in recent years (De Marco 2011). The fration of close binary CSPNe was established as 12-21% on the basis of the Optical Gravitational Lensing Experiment (OGLE) photometry in the Galactic Bulge (Miszalski et al. 2009a), consistent with the 10-15% fraction given by Bond (2000).

The growing number of binary CSPNe made it possible to compare the period distribution with the population synthesis model predictions and to study the influence of the common envelope (CE) phase of the binary evolution on the morphology of the nebulae (De Marco 2009; Miszalski et al. 2009a, b; Boffin et al. 2012).

Here, we present OGLE-II and OGLE-III observations of the Small Magellanic Cloud (SMC) PNe (Udalski et al. 1997, 2008). We report the discovery of one binary nucleus. Seven objects are confirmed to be PN mimics, including a symbiotic mira candidate.

name RA DEC V I ref diameter [arcsec] other names
LHA 115-N 4 0 34 21.99 -73 13 21.4 16.809 18.464 700 [1] 0.54 [11] LIN 16, SMP 3
LHA 115-N 5 0 41 21.68 -72 45 16.7 15.805 17.224 705 [1] LIN 32, SMP 5, [MA93] 23
LHA 115-N 6 0 41 27.75 -73 47 06.5 15.916 16.942 1213 [1] … [10] SMP 6, LIN 33, [MA93] 29
Jacoby SMC 1 0 42 28.15 -73 20 55.1 18.230 19.598 603+671 [2] SMP 7, [MA93] 39
 44 0 43 09.40 -73 08 03.7 19.803 20.487 307+655 [6]
LHA 115-N 7 0 43 25.32 -72 38 18.9 16.212 17.365 705 [1] [10] LIN 43, SMP 8, [MA93] 49
MGPN SMC 6 0 44 25.73 -73 51 39.3 20.417 20.535 1143 [7] [MA93] 73
 1 0 45 12.10 -73 18 58.0 20.487 20.445 308+682 [9]
LIN 66 0 45 20.65 -73 24 10.3 17.494 19.155 399+697 [5] 1.20 [10] J3, SMP 9, [MA93] 98
LIN 71 0 45 27.43 -73 42 14.5 15.659 14.505 1215 [5] 0.27 [10] [MA93] 104, J4
 2 0 45 36.66 -73 24 04.3 17.993 17.881 322+697 [9]
 5 0 47 39.99 -72 39 03.0 19.443 19.425 753 [9]
LHA 115-N 29 0 48 36.55 -72 58 00.9 16.715 298+723 [1] [10] SMP 11, LIN 115, [MA93] 241, J8
 6 0 49 08.24 -73 02 23.6 19.336 19.290 337+722 [9]
Jacoby SMC 9 0 49 20.11 -73 17 35.3 19.817 19.881 334+30 [3] [MA93] 290
SMP SMC 12 0 49 21.10 -73 52 58.7 18.536 17.983 354 [4] [MA93] 291
 7 0 49 35.44 -73 26 33.7 19.835 18.868 325+545 [9]
 3 0 49 47.47 -74 14 40.0 17.663 17.859 694 [8]
LHA 115-N 38 0 49 51.65 -73 44 21.4 15.382 16.913 547 [1]
LHA 115-N 40 0 50 35.09 -73 42 58.1 16.500 18.095 700 [1] 0.83 [10]
 406 0 50 52.38 -73 44 55.2 18.574 19.751 691 [6]
LHA 115-N 43 0 51 07.38 -73 57 37.6 15.341 16.444 709 [1] 0.32 [11] SMP 15, LIN 174  [MA93] 433
 12 0 51 07.82 -73 12 06.4 18.330 17.349 337+722 [9] [11]
LHA 115-N 42 0 51 27.17 -72 26 11.7 16.741 16.922 753 [1] [11] LIN 179, SMP 16, J14, [MA93] 467
LHA 115-N 47 0 51 58.13 -73 20 31.3 16.158 16.626 339+713 [1] 0.14 [10] LIN 196, [MA93] 519, J19, SMP 18
LIN 239 0 53 11.13 -72 45 07.5 16.432 17.873 321+752 [5] 0.59 [10] J20, SMP 19, [MA93] 652
 891 0 55 59.42 -72 14 00.8 19.274 20.314 712 [6]
LIN 302 0 56 19.48 -72 06 58.4 17.609 17.792 721 [5] [10] MGPN 8, [MA93] 933
LIN 305 0 56 30.77 -72 27 02.1 17.181 18.023 209+617 [5] [MA93] 943, SMP 21
 17 0 56 52.53 -72 21 02.3 19.968 19.129 661 [9]
LIN 343 0 58 42.32 -72 56 59.8 16.910 18.387 307+713 [5] [10] J26, SMP 23, [MA93] 1088
LHA 115-N 68 0 58 43.04 -72 27 16.2 18.920 288+712 [1] [MA93] 1091, LIN 339, MGPN 9
LHA 115-N 70 0 59 16.12 -72 02 00.0 16.016 16.916 721 [1] 0.38 [10] SMP 24, [MA93] 1136, LIN 347
 19 0 59 29.37 -73 39 05.9 20.943 20.641 631 [9]
LIN 357 0 59 40.51 -71 38 15.1 17.856 18.581 704 [5] SMP 25, [MA93] 1159
 20 1 00 15.11 -72 16 40.7 20.567 21.140 541 [9]
 23 1 02 00.91 -72 59 02.0 20.560 20.783 313+463 [9]
 1438 1 04 04.69 -71 37 24.9 20.320 20.421 669 [6]
LIN 430 1 04 17.90 -73 21 51.1 18.124 19.151 737 [5] [10] SMP 26, [MA93] 1454
 1709 1 09 51.84 -73 20 50.6 19.457 18.657 735 [6]
 1714 1 10 04.86 -72 45 27.5 19.342 18.411 675 [6]
SMP SMC 34 1 12 10.86 -71 26 50.6 18.584 19.771 648 [4] [10] [MA93] 1757
 1762 1 12 40.25 -72 53 46.8 20.109 20.309 90 [6] [10]
LHA 115-N 87 1 21 10.65 -73 14 34.8 15.553 16.758 637 [1] 0.45 [10] LIN 532, SMP 27, [MA93] 1884
LIN 536 1 24 11.85 -74 02 32.2 17.589 17.716 625 [5] 0.31 [11] SMP 28

References: Henize (1956), Sanduleak et al. (1978), Jacoby (1980), Sanduleak & Pesch (1981), Lindsay (1961), Meyssonnier & Azzopardi (1993), Morgan & Good (1985), Morgan (1995), Jacoby & De Marco (2002), Stanghellini et al. (2003), Shaw et al. (2006)

Table 1: List of PNe identified in the OGLE-II and OGLE-III fields in the SMC. Column gives the number of the OGLE-II (when available) and OGLE-III observations.
name RA DEC V I ref diameter [arcsec] other names
 22 0 41 09.13 -73 06 47.1 17.796 17.764 304+691 [4]
LIN 34 0 42 15.77 -72 59 55.4 16.441 16.450 315 [3] [MA93] 37
LHA 115-N 9 0 43 36.69 -73 02 26.9 15.529 15.883 317+698 [1] [MA93] 54, LIN 45
Jacoby SMC 7 0 48 05.36 -73 09 05.9 19.278 18.947 324+722 [2]
 11 0 50 52.66 -72 52 16.7 21.263 19.562 333+722 [6]
Jacoby SMC 23 0 55 30.42 -72 50 21.9 18.358 17.835 295+713 [5] … [7] [MA93] 852
Jacoby SMC 24 0 56 39.40 -72 39 07.2 17.920 16.619 295+713 [2] [MA93] 955

References: Henize & Westerlund (1963), Boroson & Liebert (1989), Morgan (1995), Haberl & Sasaki (2000), Bolatto et al. (2007), Boyer et al. (2011), Stanghellini et al. (2003)

Table 2: List of the PNe mimics showing photometric variability.

2 PNe sample

SMC PNe were selected using the SIMBAD database. We made use of the object type and coordinate search criteria and found 108 objects. The list includes PNe discovered by Lindsay (1961), Sanduleak et al. (1978), Sanduleak & Pesch (1981), Morgan & Good (1985), Morgan (1995), and Meyssonnier (1995) by means of objective-prism plates or long-slit spectroscopy. Jacoby & De Marco (2002) used narrow-band imaging, but subsequently confirmed their sample with spectroscopic follow-up. Jacoby (1980) used on-line/off-line filter photography. Their sample was spectroscopically verified by Boroson & Liebert (1989), who showed that 30% of Jacoby’s PNe candidates could not be confirmed or have other explanations.

We supplemented our list with nine PNe confirmed by Morgan (1995) and one from Sanduleak et al. (1978), which were missing from the Simbad database. We also included one object from Jacoby & De Marco (2002), classified as a He-burning asymptotic giant branch (AGB) star in the Simbad database. The total sample comprises 119 objects.

The objects were identified by visual comparison of the finding charts available in the literature with the OGLE-II and OGLE-III I-band reference images. In addition, we observed 27 objects in the SMC with the 1.0m telescope at the South African Astronomical Observatory (SAAO) with the R and filters. No emission was identified in three cases (Jacoby SMC 12, Jacoby SMC 15 and [JD2002] 3). Jacoby SMC 12 and Jacoby SMC 15 were misclassified as PNe (Boroson & Liebert 1989). [JD2002] 3 shows faint, extended nebula, probably below the sensitivity of our image (Jacoby & De Marco 2002).

We identified 52 objects in the OGLE reference images in total out of 119 candidates (Table LABEL:smcpne and 2). Though all the PNe in the sample were spectroscopically confirmed, classification of eight objects was ambiguous (Table 2). [MA93] 22 was classified as a PN (Meyssonnier 1995) even though it shows a strong continuum and a very weak [O iii] 5007Å line. Haberl & Sasaki (2000) classified it as a Be/X-ray binary. The PN nature of LIN 34 and Jacoby SMC 7 was questioned by Morgan (1995). The former object was classified as a B0-5V star by Evans et al. (2004). Jacoby SMC 23 is a young stellar object (YSO) candidate (Bolatto et al. 2007) and Jacoby SMC 24 is probably an H ii region (Boroson & Liebert 1989). Henize & Westerlund (1963) classified LHA 115-N 9, LHA 115-N 61, and LHA 115-N 68 as diffuse nebulae. But we classify LHA 115-N 68 as a PN, since it shows a prominent [O iii] 5007Å line (Leisy & Dennefeld 2006). [JD2002] 11 was classified as an extreme AGB star candidate on the basis of the infrared colour-magnitude diagram (Boyer et al. 2011).

Seven known PN mimics show photometric variability. We present here the spectroscopy and photometry of these objects in addition to the genuine PNe to study their characteristics (which may be of interest to other research groups) in an endeavour to understand their nature, before ultimately rejecting them from the PN sample.

3 Observations

We used the V- and I-band photometry of the SMC collected by the OGLE-III project. OGLE-II photometry was available for some of the objects as well. The V-band photometry was obtained only intermittently, while I-band observations were made frequently. The I-band filter had a wide long-wavelength wing, quite different from the sharp drop of transmission at about 9000Å of the standard I-bandpass (Udalski et al. 2002). The data were taken only under good seeing conditions. The median seeing during the observations of the SMC was equal to 1.”2 (Udalski et al. 2008).

The photometry of some objects is affected by systematic effects, which hampers the time-series analysis. This problem was previously reported by Miszalski et al. (2009a) for CSPNe suffering from strong nebular contamination. We examined OGLE I-band images of our objects. Only two of them are clearly resolved: LHA 115-N 9 and LHA 115-N 61, both classified as diffuse nebulae (Section 2). Some other objects have full width at half maximum (FWHM) that is somewhat larger than that of the field stars.

Eight objects were found to be variable. We performed spectroscopic observations of five variables with the Southern African Large Telescope (SALT), and three variables were observed with the Radcliffe 1.9m telescope at SAAO (Table LABEL:salt). The SALT low-resolution spectra were obtained with the 900 l/mm grating. The slit was 1 arcsec wide and 8 arcmin long. The spectral coverage was 4350-7400Å with the spectral resolution of about at the central wavelength. A 2 arcsec wide slit and the 300 l/mm grating was used for the 1.9m telescope, resulting in the spectral resolution of about 2500 at the central wavelength. The spectral coverage was set to 3500-7500Å. All the spectra were flux calibrated.

date telescope exptime name
2012-09-17 SALT 120 Jacoby SMC 24
2012-09-19 SALT 120+1200 Jacoby SMC 23
2012-09-19 SALT +1200 Jacoby SMC 1
2012-10-10 SALT 120+1200 Jacoby SMC 1
2012-11-25 SALT 900 Jacoby SMC 7
2012-11-26 SALT 900
2013-07-17 Radcliffe LHA 115-N 61
2013-07-21 Radcliffe LIN 34
2013-07-21 Radcliffe [MA93] 22
Table 3: Spectroscopic observations of objects showing photometric variability.

4 Results

We analysed the OGLE-II and OGLE-III I-band photometry using the program Period04 (Lenz & Breger 2005) and the analysis of variance (ANOVA) method (Schwarzenberg-Czerny 1996). We identified six periodic variables (including one with a period of about 4.5 years) and two irregular variables. The phased light curves of the all periodic variables are presented in Figure 1. The irregular light curves and periodic light curves with periods long enough to be legible are shown in Figure 2. The spectra of all the variable objects but [MA93] 22 are presented in Figure 3. All the spectra are available at CDS (Table 4).

Out of the total sample of eight variable objects detected, we considered seven objects to be PNe mimics. They have previously been suggested to be PNe mimics or were considered as objects of uncertain nature. One remaining (periodic) variable is most likely a genuine binary CSPN.

4.1 Variable stars

LIN 34 has a period of day (Figure 1). It is catalogued as a PN, but Meyssonnier & Azzopardi (1993) classified it as a very low excitation (VLE) object. The spectrum of LIN 34 is characterised by strong continuum and lack of obvious [O iii] 5007Å emission (Figure 3). The observed wavelength of the emission lines corresponds to a radial velocity of about , thus LIN 34 appears to be a pulsating Galactic halo Be star (Greenstein & Sargent 1974).

Figure 1: Phased OGLE I-band light curves of the SMC periodic variables.

[MA93] 22 was classified as a PN by Meyssonnier (1995), although its spectrum revealed a strong continuum and a very weak [O iii] 5007Å line. The object shows variability on a timescale of about 1630 days (Figure 2). Three maxima were covered by the OGLE-II and -III phase observations. The last maximum is fainter than the first two maxima by about 0.1 mag, which affected the appearance of the phased light curve of the object (Figure 1). The object showed only a weak emission in a very noisy 1.9m telescope spectrum, not shown in Figure 3. [MA93] 22 was proposed as the optical counterpart of an X-ray binary afterwards (Haberl & Sasaki 2000). It is clearly detected in the 2MASS K band, but not detected in the J and H bands. The near-IR emission lines may significantly contribute to the brightness of the object in the K band (Naik et al. 2012). The optical variability could correspond to the orbital period of the binary.

[JD 2002] 11 has a period of days (Figure 1). The [O iii] 5007Å emission line is detected in the spectrum, exceeding the line in intensity (Figure 3). [JD 2002] 11 probably is a symbiotic system. The 215.3 day period may be due to the pulsation of the cool component, while the hot component is responsible for the ionization of the nebula. However, the quality (and wavelength range) of the spectrum did not allow us to unveil the spectral signature of the cool component.

Near-infrared photometry allows one to separate genuine PNe from symbiotic miras (Schmeja & Kimeswenger 2001). PNe are located in a well-defined region in the IJK two-colour diagram. We searched for the Two Micron All Sky Survey (2MASS) photometry of all the PNe identified in the SMC (Skrutskie et al. 2006). Infrared colours confirm that [JD 2002] 11 is a symbiotic mira and not a PN (Figure 4). If the classification is correct, then [JD2002] 11 would be only the eight symbiotic star discovered in the SMC (Belczyński et al. 2000; Oliveira et al. 2013).

Jacoby SMC 7 has a period of days (Figure 1). The object was disclaimed to be a genuine PN by Boroson & Liebert (1989). It is located in an extended [O iii] 5007Å and H i emission region of the supernova remnant (SNR) [FBR2002] J004806-730842. The spectrum of Jacoby SMC 7 contains H i lines, but the [O iii] 5007Å emission was extracted with the diffuse background emission of the SNR (Figure 3). Mennickent et al. (2002) observed a group of Be stars characterized by periods longer than 17 days. Jacoby SMC 7 may be a new member of this group.

Jacoby SMC 23 lies on the edge of an open cluster OGLE-CL SMC 104. The object experienced a sudden flux increase during the first season of the OGLE-III operation (Figure 2). No variability was found before that event. No significant colour change accompanied the outburst. The object shows wide line wings in the spectrum (Figure 3), which were previously reported by Stanghellini et al. (2003). Jacoby SMC 23 is most likely a YSO experiencing an increase of the accretion rate. It may be an optical counterpart of the YSO candidate S3MC J005530.39-725021.87 (Bolatto et al. 2007).

The spectrum of Jacoby SMC 24 shows only and lines (Figure 3). The light curve reveals irregular or semi-regular variations (Figure 2). We do not consider this object as a genuine PN. The nature of this PN candidate was previously debated by Morgan & Good (1985) and the object was not recovered by Jacoby & De Marco (2002). The light curve showing stochastic variability is similar to the light curves of most of the Be stars in the SMC (Mennickent et al. 2002).

LHA 115-N 61 shows eclipses lasting for three days with a period of about 138 days (Figure 1). The light curve shows an additional 183-day variability (half a year) of instrumental origin (Figure 2). There is no OGLE-II photometry of the source available since it was resolved by the pipeline. The object was suggested to be a diffuse nebula by Henize & Westerlund (1963), because it was brighter than other SMC PNe and resolved. The observed [O iii] 5007Å/H flux ratio is of the order of unity, which may suggest a PN excited by a young central star. However, in that case the nebula would be compact and unresolved. The nebula is most likely excited by an eclipsing binary system of O V type stars. The depth of the eclipses may be supressed by the nebular contribution.

Figure 2: OGLE-II and -III photometry of the variable objects with periods long enough to be readily presented.
Figure 3: Spectra of the variable objects.
Figure 4: IJK two-colour diagnostic diagram for the Galactic and SMC PNe. Open circles show galactic objects classified as PNe, while triangles denote known symbiotic miras. Crosses denote the SMC PNe. Names of miras and suspected miras are labelled (Schmeja & Kimeswenger 2001).

Jacoby SMC 1 is a binary CSPN with a period of days assuming the irradiation efect to be the cause of the variations, or twice that period in the case of ellipsoidal variations (Figure 1). The observed amplitude of the variations is about 0.3 magnitude, but may be supressed by the nebular contribution to the I brightness. The spectrum, showing strong [O iii] 5007Å emission and faint, blue continuum, strongly supports that the object is a genuine PN. However, we did not detect any irradiation lines coming from the cool component (Figure 3), which suggests that the ellipsoidal effect is the cause of the variability.

4.2 Binary fraction

Figure 5: OGLE I-band transmission curve and an example PN and CS spectum.

One binary CSPN was found in the SMC sample of 45 PNe. This suggests that only about 2% of SMC PNe may be the influenced by the CE phase, with the upper limit of about 7% (with 90% certainty, assuming 100% detection efficiency). This is lower than the binary fraction of 12 to 21% obtained for the Galactic Bulge (Miszalski et al. 2009a). However, the observed binary fraction in the SMC is subject to small statictics and strong selection effects, some of which may be distance dependent. Below we list and assess the influence of the selection effects.

To determine the binary fraction, the sample must be first corrected for the PN mimics. As explained in Section 2, all the PNe in the sample were spectroscopically confirmed. Usually the SMC PNe were identified on the basis of the presence of the [O iii] 5007Å line, lack of stellar continuum, and small angular dimensions (unresolved sources). Contamination of diffuse nebulae, Strömgren spheres in the interstellar medium, or compact H ii regions is possible. About 10% of the sample show an [O iii] 5007Å/H flux ratio of the order of unity and may be unresolved diffuse nebulae.

Detection of photometric variations is possible only when the stellar continuum contributes significantly to the total brightness of the object in the I-band. The nebular lines and continuum can produce the main or even the entire contribution in the I-band in many spatially unresolved SMC PNe, supressing the amplitude of a binary central star. The HST observations showed that most of the SMC PNe do not exceed 1 arcsec in diameter (Stanghellini et al. 2003; Shaw et al. 2006) and would not be resolved in the OGLE images.

To assess the relative contribution of the stellar continuum to the total I-band flux of an unresolved PN, we used an exploratory PN model offered by the test suite distributed with the code Cloudy (Ferland et al. 1998). The model assumed that the 50,000 K blackbody spectrum excites the ionization bounded nebula with reduced elemental abundances. The OGLE I-band transmission curve is shown along with the central star and nebular spectrum in Figure 5. The [S iii] 9069 and 9532Å lines contribute to the calculated brightness of the object in the nonstandard OGLE I-band filter (Udalski et al. 2002). The total nebular contribution is six time higher than the contribution of the central star in the I-band (18.6 magnitude at the distance to the SMC). The nebular contribution would be much lower for a cooler central star and a density bounded PN.

We plot the histogram of the colours of the identified PNe in Figure 6. Most of the objects have colours different from , expected for unreddened CSPN (Ciardullo et al. 1999), even those for which the central star was easily detected in the HST observations. The diagram is only slightly affected by reddening (an average of 0.07 for the SMC, Haschke et al. 2011).

Figure 6: Histogram of the V-I colours of all identified SMC PNe.
Figure 7: Histogram of the I-band brightness of all identified SMC PNe. The hatched histogram of the binary CSPNe in the Galactic Bulge is overplotted (Miszalski et al. 2009a).

Central stars were detected in about 75% of the SMC PNe (Villaver et al. 2004). The nebular contribution could be significant in some of them. In 25% of cases no central star was detected, with the PN contributing all the light to the I-band. The inclusion of such objects in our sample can lower the derived binary fraction by about 25%.

The SMC CSPNe are expected to be fainter than their Galactic Bulge counterparts because of the difference in distance. This could lower the detection efficiency of the binary CSPNe. Assuming a similar intrinsic I-band brightness distribution of the SMC and the Galactic Bulge CSPNe populations, the observed distributions would be separated by about 4.7 mag. However, Galactic Bulge PNe are observed in the fields close to the Galactic centre, typically reddened by of 1 or more (Nataf et al. 2013) contrary to the only slightly reddened SMC PNe (Haschke et al. 2011). In addition, SMC PNe are more likely to be unresolved and contribute to the I-band brightness. This would reduce the separation of both distributions.

The distributions of the I-band brightness of the SMC PNe and the Galactic Bulge PNe known to harbour binary CSPNe (Miszalski et al. 2009a) are compared in Figure 7. Bulge PNe are brighter on average only by about 1.5 mag from the SMC PNe, and their brigntess distribution is much narrower. However, neither of the distributions can be treated as representative for the whole PN population. Very bright PNe, avoided by the OGLE-III pipeline, as well as very faint PNe may be underrepresented in the Galactic Bulge histogram. Some of the SMC PNe fall below the completness limit of 21 mag for the OGLE-III survey in the I-band (Udalski et al. 2008).

The sensivity of our search depends on the apparent brightness of the object and the data sampling. Only one object in our sample has fewer than 100 datapoints. More than half of the PNe identified in the SMC fields are fainter than 18 mag (Figure 7). However, the sensitivity limit for an 18 mag star observed 600 times is about 0.01 mag for a sinusoidal light curve. This is sufficient to detect most of the CSPNe with periods of up to 1-2 weeks, unless the binary system parameters were very unfavourable (De Marco et al. 2008). However, this requires at least one of the binary components to be observable at the distance to the SMC.

An M V type companion of the CSPN would be below the OGLE detection limit at the distance to the SMC (fainter than 26 mag in the I-band). Binary CSPNe discovered purely because of irradiation effects constitute almost half of the systems detected by Miszalski et al. (2009a). A detection of the CSPN binaries showing ellipsoidal variations and eclipsing binaries would be feasible, although the detection efficiency would be reduced by the nebular contribution.

Taking all the selection effects into account, the number of detected binaries toward the SMC may be reduced by 80% compared with that of the Galactic Bulge. The binary fraction corrected for the selection effects may be as high as 10% or higher.

4.3 Period distribution

The period distribution of the binary CSPNe in the Galaxy has a maximum of about 0.4 day with only few systems with periods longer than one day (Miszalski et al. 2011). The distribution of the close WD-MS CSPNe binaries appears to agree well with the WD-MS post CE systems (Hillwig 2011). The latter distribution shows a lack of systems with periods longer than 5-10 days (Nebot Gómez-Morán et al. 2011).

The period of Jacoby SMC 1 is three to six times longer than the maximum of the distribution for binary CSPNe in the Galaxy. The probability of finding of a CSPN binary with such a long period is relatively low, but not negligible. We cannot rule out the possibility that this reflects the real difference in the period distributions for the Galactic and SMC binary CSPNe.

We cannot confirm whether Jacoby SMC 1 contains a WD-MS or a double degenerate binary. The period distribution for the Galactic double degenerate binaries may have maximum shorter than 0.4 day.

The post-CE period distribution depends primarily on the binding parameter and the initial mass-ratio distribution. The cutoff for the periods above a few days in the distribution of the Galactic binary CSPNe suggests the , otherwise a tail towards longer orbital periods of up to 1000 days would be expected (Miszalski et al. 2009a). The binding parameter of an envelope of a giant star does not show a strong dependance with metallicity (Loveridge et al. 2011), thus may be equal to 0.3 for the SMC as well.

The maximum of the period distribution depends on the assumed initial mass ratio. Zorotovic et al. (2011) have found that systems containing high-mass secondaries tend to have longer post-CE orbital periods for a Galactic sample for post-CE binaries. The amplitude of the photometric variability increases with the secondary mass, but decreases with the orbital separation. Thus binaries with high-mass secondaries may have similar amplitudes (and the detection efficiency) to the systems with low-mass companions. It is unclear, however, why would the SMC binaries have more massive secondaries.

Reduced mass loss during the AGB phase may lead to somewhat more massive central stars of PNe (Villaver et al. 2007), but no correlation between the WD mass and the periods of the post-CE systems was found for CO-core WDs (Zorotovic et al. 2011).

5 Summary

We discovered only one binary CSPN in the SMC. The actual binary fraction of the CSPNe in the SMC may be similar to the value obtained for the Galactic Bulge PNe taking the selection effects into account. The sample is too small to derive firm conclusions concerning the period distribution of the post-CE binary CSPNe in the SMC.

In addition, observations allowed us to constrain the nature of some known PNe mimics. Three of them are probably Be stars and one is an YSO. The likelihood that [MA93] 22 is an optical counterpart of an Be/X-ray binary has increased. We discovered a new symbiotic star candidate.

Acknowledgements.
This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This work was financially supported by NCN of Poland through grants No. 2011/01/D/ST9/05966 and 719/N-SALT/2010/0. The OGLE project has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 246678. Some of the observations reported in this paper were obtained with the Southern African Large Telescope (SALT), proposal 2012-1-POL-010, P.I. M. Hajduk. We thank the anonymous referee for the comments on the paper.

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