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Long-term variability survey of the old open cluster NGC 6791

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a Long-term variability survey of the old open cluster NGC 6791. B. J. Mochejska 1, K. Z. Stanek Harvard-Smithsonian Center for Astrophysics, 60Garden St., Cambridge, MA 02138bmochejs@cfa.harvard.edu,kstanek@cfa.harvard.eduJ. Kaluzny Copernicus Astronomical Center, Bartycka 18, 00-716Warszawa jka@camk.edu.plABSTRACT We present the results of a long-term variability survey of the old open cluster NGC 6791. The BV I observations, collected over a time span of 6years, were analyzed using the ISIS image subtraction package. The main target of our observations were two cataclysmic variables B7and B8. We have identifiedpossible cycle lenghts of about 25and 18days for B7and B8, respectively. We tentatively classify B7as a VY Scl type nova-like variable or a Z Cam type dwarf nova. B8is most likely an SS Cygni type dwarf nova. We have also extracted the light curves of 42other previously reported variable stars and discovered seven new ones. The new variables show long-period or non-periodic variability. The long baseline of our observations has also allowed us to derive more precise periods for the variables, especially for the short period eclipsing binaries. Subject headings:open clusters and associations:individual:NGC 6791–cataclysmic variables –binaries:eclipsing –stars:variables:other –color-magnitude diagrams 1. Introduction The open cluster NGC 6791is unique in many ways. At an age of about 8Gyr it is believed to be the oldest open cluster in the Galaxy (Chaboyeret al. 1999, Kaluzny &Rucinski 1995). It is also

probably the most metal-rich, with [Fe/H]esti-mates ranging from 0.1to 0.5dex (Frielet al. 2002, Chaboyer et al. 1999, Peterson &Green 1998, Kaluzny &Rucinski 1995). NGC 6791pos-sesses two of the three cataclysmic variables (CVs)found in open clusters (Kaluznyet al. 1997), with the third residing in M67(Gillilandet al. 1991) 2. Variability in NGC 6791was firststudied by Kaluzny &Rucinski (1993),who discovered 17variable stars, among them one CV candidate. Rucinski, Kaluzny &Hilditch (1996)reported ad-

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