Peremennye Zvezdy (Variable Stars) 31, No. 1, 2011 Received 7 January; accepted 19 January.

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GSC 2576-02071 and GSC 2576-01248: two Algol-type eclipsing binaries studied using CCD observations and historical photographic data

K. V. Sokolovsky^1,2, S. V. Antipin^3,4, A. V. Zharova³, S. A. Korotkiy⁵

1. Max-Planck-Institute für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

2. Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia

3. Sternberg Astronomical Institute, University Ave. 13, 119992 Moscow, Russia

4. Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya Str. 48, 119017 Moscow, Russia

5. Ka-Dar Public Observatory, Barybino, Domodedovo District, Moscow Region, Russia

1. GSC 2576-01248: a short-period Algol-type binary with O'Connell effect

GSC 2576-01248 = T-CrB0-04444 = 2MASS 16113251+3306054 (16113251 +330605 4, J2000; Skrutskie et al. 2006) was discovered as an eclipsing binary by Devor et al. (2008) during the Trans-atlantic Exoplanet Survey (TrES; Alonso et al. 2004). It was also independently detected by the authors using archival photographic data during initial tests of our digitized photographic plate analysis pipeline (e.g., Sokolovsky 2006; Kolesnikova et al. 2010). The digitized photographic plates were originally obtained in 1973-1975 with the 40-cm astrograph of the Crimean Laboratory of Sternberg Astronomical Institute (series "A" of the Moscow plate collection, field  CrB). To assess the quality of the photographic photometry obtained with our pipeline, in 2006 we have conducted V-band CCD observations of the binary using the 50-cm Maksutov telescope (AZT-5) of the Crimean Laboratory. The CCD observations were reduced in the standard way involving dark-frame and flat-field corrections and aperture photometry of the target star, the comparison (TYC 2576-2051-1, V=10.76, Høg et al. 2000) and check stars in the field of view. A typical accuracy of individual CCD photometric measurements is while the typical accuracy of our photographic photometry for a - star was found to be .

Our series of CCD frames was checked for presence of other variable objects in the field using the VaST software (Sokolovsky and Lebedev 2005), which resulted in the discovery of another eclipsing binary, GSC 2576-02071, just away (see the following section). The effort to determine the period of the newly discovered binary using observations with a number of telescopes (Table 1) in 2006-2007 has resulted in a high-quality lightcurve also for GSC 2576-01248 since both stars fit in the same field of view for all the instruments used (Fig. 1).

Observations obtained with different instruments were combined into a single lightcurve by shifting individual lightcurves to match the average out-of-eclipse brightness to that observed with AZT-5. The eclipse depth is the same for both -band and unfiltered lightcurves (Fig. 2) thanks to the spectral response curve of the blue-sensitive KAI-2020M CCD chip used in the ST2000XM camera, which reaches its peak quantum efficiency in the 410-550 nm range around the  band.

The combined lightcurve of GSC 2576-01248 was analyzed with the Lafler & Kinman (1965) method, which is well-suited for eclipsing binaries of the Algol type, such as the two variable stars presented in this paper. Fig. 3 shows the fine structure of the most significant peak on the Lafler & Kinman periodogram with and without the archival photographic data included in the analysis. Thanks to the large time baseline (photographic observations were obtained in 1973-1975 while the CCD observations were obtained in 2005-2007), the addition of less-precise photographic data to the high-accuracy CCD observations provides a significant improvement in the periodogram peak localization resulting in the following light elements:

The phased lightcurves folded with the above period are presented in Fig. 4. The eclipses are partial and last for about 2 hours. The primary eclipse is deep and the secondary eclipse is deep in the  band.

Fig. 5 shows the phased lightcurve for the two consecutive observing seasons, 2006 and 2007. The presence of a prominent ( ) time-variable O'Connell effect (O'Connell 1951, Davidge & Milone 1984) is evident from the plot. The difference in maxima brightness (and eclipse depth) seems to be caused by a wide dip, slowly travelling backwards through the phased lightcurve. The effect may be explained by the presence of a dark starspot on one of the binary system components that rotates asynchronously (slightly slower) with respect to the orbital motion.

Times of individual minima were measured from the CCD lightcurves by applying the standard Kwee & van Woerden (1956) method. A relatively large number of minima observed over the long time baseline (Table 2) provides an opportunity to search for possible period changes. From the plot (Fig. 6) it is evident that the long-term behavior of the system is consistent with the orbital period being the same in 1973-1975 and 2005-2007. The estimated (also following Kwee & van Woerden 1956) errors of minima time determination cannot account for the observed scatter of measurements around ; however, we note that these error estimations should be treated with caution since they do not account for any systematic difference in the shape of the descending and ascending branches of the eclipse lightcurve (real or caused by uncorrected systematic effects in photometry) or for the sensitivity of the method to the exact choice of eclipse boundaries, which needs to be set manually for the time of minimum determination. For photographic observations, no time series were recorded during eclipses. The few faintest data points on the photographic lightcurve were taken as the best available minima time estimates in that era. The total uncertainty of the photographic times of minima determination was set to a typical exposure time used for the plates ( min).

2. GSC 2576-02071: a long-period Algol-type binary with an eccentric orbit

GSC 2576-02071 = T-CrB0-03105 = 2MASS 16111795+3307131 (16111796 +330713 2, J2000; Skrutskie et al. 2006) was discovered as an Algol-type eclipsing binary during our CCD observations of GSC 2576-01248. After the initial detection of an eclipse in summer 2006, it took a significant amount of observing time with a number of telescopes (Table 1) and the addition of the TrES archival data to accurately determine the binary's orbital period:

The archival photographic data provide no additional constrains on the period because of their low accuracy, small variability amplitude of GSC 2576-02071, and lacking photographic data around the primary eclipse. Therefore, only CCD data were used for the period determination. We have employed the same data analysis and period search technique as for GSC 2576-01248 (described in the previous section). The times of minima observed for GSC 2576-02071 are listed in Table 3.

Fig. 7 presents the phased lightcurve of GSC 2576-02071. The secondary eclipse phase is , indicating a significant orbital eccentricity. Additional observations are desirable to search for signs of an apsidal motion or period change in this system. The eclipses are partial and last for about 6 hours (Fig. 8). The primary eclipse is deep while the secondary eclipse is deep. The scatter of the out-of-eclipse lightcurve (excluding the photographic data) is characterized by , which is consistent with the expected accuracy of the combined multi-instrument CCD dataset.

3. Conclusions

An initial investigation of two poorly studied eclipsing binary stars, GSC 2576-02071 (previously unknown) and GSC 2576-01248 (discovered by Devor et al. 2008), based on the archival photographic and modern CCD data is presented. In the case of GSC 2576-01248, the addition of photographic observations from 1973-1975 provides a significant improvement in the accuracy of period determination over the CCD-only dataset obtained in 2005-2007 thanks to the long time baseline between the observation epochs. Comparison of the archival photographic data with modern-day CCD photometry also provides an opportunity to search for possible long-term period changes, which, however, were not detected for the two binaries described here. The secondary minimum of GSC 2576-02071 is shifted from the phase . This binary system is a promising apsidal-motion candidate.

Acknowledgments: We are deeply grateful to Jonathan Devor, Francis O'Donovan, Georgi Mandushev, and David Charbonneau of the TrES team for providing their observations of GSC 2576-02071 and GSC 2576-01248. We would like to thank S. V. Nazarov for the opportunity to use CrAO's 24-cm telescope and V. P. Goranskij for providing his lightcurve analysis software. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the UMass/IPAC-Caltech, funded by the NASA and the NSF. This work has made use of the Aladin interactive sky atlas, operated at CDS, Strasbourg, France, the International Variable Star Index (VSX) operated by the AAVSO, and NASA's Astrophysics Data System. K.S. is supported by the International Max-Planck Research School (IMPRS) for Astronomy and Astrophysics at the universities of Bonn and Cologne.

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