Long-term $Bvri$ light curves of 5 pre-main sequence stars in the field of "Gulf of Mexico"

V521 Cyg was discovered by Herbig (1958) as H$\alpha$ emission line star and classified as TTS by Herbig & Bell (1988), Fernandez et al. (1995) and Laugalys et al. (2006). Terranegra et al. (1994) and Armond et al. (2011) classified the star as CTTS. Grankin et al. (2007) showed long-term photometric curve of V521 Cyg and concluded that the star exhibit unusual colour behavior with a blue turnaround at minimum brightness, probably caused from scattered light during partial occultation of the stellar photosphere by circumstellar material. Poljančić Beljan et al. (2014) determined the period of V521 Cyg, which is found to be 503 days.

The $BVRI$ light curves of V521 Cyg from all our CCD observations (Poljančić Beljan et al. 2014, and the present paper) are shown in Fig. 2. On the figure, circles denote photometric data acquired with the 2-m RCC telescope; diamonds - the photometric data taken with the 1.3-m RC telescope; triangles - the photometric data collected with the 50/70-cm Schmidt telescope, and squares - the photometric data obtained with the 60-cm Cassegrain telescope.

The photometric results of our recent CCD $BVRI$ observations of V521 Cyg are summarized in Table 2. The columns contains Julian Date (J.D.) of observations, measured $IRVB$ magnitudes of the star, telescope and CCD camera used.

The brightness variations of V521 Cyg in the different bands during the period of our study (1997-2014) are 11.78 – 12.85 mag for $I$-band, 12.55 – 14.05 mag for $R$-band, 13.32 – 14.89 mag for $V$-band, and 14.49 – 16.69 mag for $B$-band. The observed amplitudes are 1.07 mag for $I$-band, 1.50 mag for $R$-band, 1.57 mag for $V$-band and 2.20 mag for $B$-band in the same period.

Figure 2 shows the long-term photometric variability of V521 Cyg and the presence of seven deep declines in brightness observed in all bands, as follows: two deep declines observed in 2002, one in 2010, one decline in the beginning of 2011, one lasting longer decline in 2012, one at the end of 2013, and one very deep decline in 2014. During these declines, the brightness of the star faded for more that 1 mag. There may be others deep declines of brightness, but they were not registered in our photometric study. The declines in brightness of V521 Cyg likely are caused by partial and irregular obscuration of the star by circumstellar material and V521 Cyg can be classified as CTTS with evidences for Type V (UXor-type) of variability. The results obtained in our study support the results of Grankin et al. (2007) about the causes of variability of V521 Cyg. But the different shape of the observed declines gives grounds to predict a variety of eclipse reasons.

The variability of V752 Cyg was discovered by Erastova & Tsvetkov (1978) and confirmed by Kohoutek & Wehmeyer (1997). For the first time the long-term $BVRI$ photometric light curves of V752 Cyg were published in Poljančić Beljan et al. (2014).

The recent photometric results from our CCD $BVRI$ observations of the star are summarized in Table 3. The columns have the same contents as in Table 2. The $BVRI$ light curves of V752 Cyg during the period of all our observations (Poljančić Beljan et al. 2014, and the present paper) are plotted in Fig. 3. The symbols used for different telescopes are as in Fig. 2. The figure shows very strong and irregular variability, without evidence of periodicity. The variations in brightness of V752 Cyg in the different bands durind the period of our photometric study (2006-2014) are 14.28 – 15.55 mag for $I$-band, 14.89 – 16.67 mag for $R$-band, 15.38 – 17.71 mag for $V$-band, and 16.02 – 18.89 mag for $B$-band. The observed amplitudes are 1.27 mag for $I$-band, 1.78 mag for $R$-band, 2.33 mag for $V$-band and 2.87 mag for $B$-band in the same period.

The irregular variations in brightness of V752 Cyg and the large photometric amplitudes (up to 2.33 in $V$-band) are indication, that the star can be classified as CTTS with Type II of variability. The observed strong photometric variability could be caused by the highly variable accretion from the circumstellar disk onto the stellar surface.

V1539 Cyg was discovered by Herbig (1958) as H$\alpha$ emission line star and classified as TTS by Herbig & Bell (1988). Armond et al. (2011) classified V1539 Cyg as CTTS.

The $BVRI$ light curves of V1539 Cyg from all our CCD observations (Poljančić Beljan et al. 2014, and the present paper) are shown in Fig. 4. The symbols used for different telescopes are as in Fig. 2. Table 4 presented our recent CCD $BVRI$ observations of the star. The columns have the same contents as in Table 2.

The variations in brightness of V1539 Cyg in the different bands during the period of our photometric study (1997-2014) are 13.34 – 13.68 mag for $I$-band, 14.28 – 14.72 mag for $R$-band, 15.28 – 15.79 mag for $V$-band, and 16.56 – 17.26 mag for $B$-band. The observed amplitudes are 0.34 mag for $I$-band, 0.44 mag for $R$-band, 0.51 mag for $V$-band and 0.70 mag for $B$-band in the same period. The irregular photometric variability with relatively small amplitudes likely are caused by rotating cool and hot spots on the stellar surface. Therefore, the variability of V1539 Cyg can be attributed to Type I and III.

The variability of V1716 Cyg was discovered by Erastova & Tsvetkov (1978). Findeisen et al. (2013) described two burst of the star, which are separated by 35 days. The first burst lasting 5-20 days, and second burst lasting 3 days. Poljančić Beljan et al. (2014) determined the period of V1716 Cyg, which is found to be 4.15 days. The periodicity is stable for a period of several years and is probably connected with the stellar rotation.

The $BVRI$ light curves of V1716 Cyg from all our CCD observations (Poljančić Beljan et al. 2014, and the present paper) are shown in Fig. 5. The symbols used for different telescopes are as in Fig. 2. The photometric results of our CCD $BVRI$ observations of the star are summarized in Table 5. The columns have the same contents as in Table 2.

The variations in brightness of V1716 Cyg in the different bands during the period of our photometric study (1997-2014) are 14.43 – 15.36 mag for $I$-band, 15.28 – 16.68 mag for $R$-band, 16.14 – 17.93 mag for $V$-band, and 17.05 – 19.55 mag for $B$-band. The observed amplitudes are 0.93 mag for $I$-band, 1.40 mag for $R$-band, 1.79 mag for $V$-band and 2.50 mag for $B$-band in the same period.

Figure 5 shows four eruptive events of V1716 Cyg with large amplitudes. The first eruption was detected on March 2006 with an amplitude reaching up to 2.3 mag in $B$-band, the second one was detected on June 2011 with an amplitude of 1.6 mag in $B$-band, the thirdly eruption is detected on September 2012 with an amplitude of 0.7 in $B$-band, and the fourthly eruption is detected on July 2013 with an amplitude about 0.9 mag in $B$-band. These irregular flares can be explain with short-lived accretion-related events at the stellar surface or as flares from UV Ceti type. Other irregular variations of the brightness of V1716 Cyg with smaller amplitudes likely are caused by rotating hot and cool spots on the stellar surface. These results are indication that V1716 Cyg is possibly CTTS with variability of Type I, II and III.

V2051 Cyg was discovered as flare star by Parsamian et al. (1994). The star showed burst event on September 7, 1977 with amplitude $\ge$ 4 mag in $U$-band (Parsamian et al. 1994). Poljančić Beljan et al. (2014) determined the period of the star, which is found to be 384 days.

The $BVRI$ light curves of V2051 Cyg from all our CCD observations (Poljančić Beljan et al. 2014, and the present paper) are shown in Fig. 6. The symbols used for different telescopes are as in Fig. 2. The photometric results of our CCD $BVRI$ observations of the star are summarized in Table 6. The columns have the same contents as in Table 2. During the period of our observations other flares of the star are not registered, except a few low amplitude increases in brightness in $B$-band.

The variations in brightness of V2051 Cyg in the different bands during the period of our photometric study (2000-2014) are 13.84 – 14.02 mag for $I$-band, 15.39 – 15.68 mag for $R$-band, 16.04 – 16.77 mag for $V$-band, and 17.80 – 18.35 mag for $B$-band. The observed amplitudes are 0.18 mag for $I$-band, 0.29 mag for $R$-band, 0.73 mag for $V$-band and 0.55 mag for $B$-band in same period.

On the basis of the observed flare with amplitude $\ge$ 4 mag in $U$-band by Parsamian et al. (1994), the available literature data and the registered in our study low amplitude variability, we classify V2051 Cyg as low-mass WTTS. Therefore, the variability of the star can be attributed to Type IV.

We are continuing to collect regular $BVRI$ photometric observations of the field of "Gulf of Mexico" in order to classify the stars from our study with higher accuracy. For this purpose of obtaining of spectral observations of the young stellar objects in the field of "Gulf of Mexico" will be of great importance.