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

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aLong-termvariabilitysurveyoftheoldopenclusterNGC6791.B.J.Mochejska1,K.Z.StanekHarvard-SmithsonianCenterforAstrophysics,60GardenSt.,Cambridge,MA02138bmochejs@cfa.harvard.edu,kstanek@cfa.harvard.eduJ.KaluznyCopernicusAstronomicalCenter,Bartycka18,00-716Warszawajka@camk.edu.plABSTRACTWepresenttheresultsofalong-termvariabilitysurveyoftheoldopenclusterNGC6791.TheBVIobservations,collectedoveratimespanof6years,wereanalyzedusingtheISISimagesubtractionpackage.ThemaintargetofourobservationsweretwocataclysmicvariablesB7andB8.Wehaveidentifiedpossiblecyclelenghtsofabout25and18daysforB7andB8,respectively.WetentativelyclassifyB7asaVYScltypenova-likevariableoraZCamtypedwarfnova.B8ismostlikelyanSSCygnitypedwarfnova.Wehavealsoextractedthelightcurvesof42otherpreviouslyreportedvariablestarsanddiscoveredsevennewones.Thenewvariablesshowlong-periodornon-periodicvariability.Thelongbaselineofourobservationshasalsoallowedustoderivemorepreciseperiodsforthevariables,especiallyfortheshortperiodeclipsingbinaries.Subjectheadings:openclustersandassociations:individual:NGC6791–cataclysmicvariables–binaries:eclipsing–stars:variables:other–color-magnitudediagrams1.IntroductionTheopenclusterNGC6791isuniqueinmanyways.Atanageofabout8GyritisbelievedtobetheoldestopenclusterintheGalaxy(Chaboyeretal.1999,Kaluzny&Rucinski1995).Itisalso

probablythemostmetal-rich,with[Fe/H]esti-matesrangingfrom0.1to0.5dex(Frieletal.2002,Chaboyeretal.1999,Peterson&Green1998,Kaluzny&Rucinski1995).NGC6791pos-sessestwoofthethreecataclysmicvariables(CVs)foundinopenclusters(Kaluznyetal.1997),withthethirdresidinginM67(Gillilandetal.1991)2.VariabilityinNGC6791wasfirststudiedbyKaluzny&Rucinski(1993),whodiscovered17variablestars,amongthemoneCVcandidate.Rucinski,Kaluzny&Hilditch(1996)reportedad-

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