The Globular Cluster M15. I. Identification, Discovery, and Period Determination of Variable Stars

Abstract

We present new $BVI$ CCD photometry for variables in the globular cluster M15. Our photometry was obtained using both the image subtraction package ISIS and DAOPHOT/ALLFRAME. The data were acquired in 2001 on two observing runs on 11 observing nights using the 2-m telescope of the Bulgarian National Astronomical Observatory ``Rozhen'' with a Photometrics CCD camera. For 40 previously known variables, we present a period for the first time, and improved periods were obtained for many previously known variables. Fourteen new variables are reported. We present updated Bailey diagrams for the cluster and discuss its Oosterhoff classification. Although many of M15's RRab pulsators fall at an intermediate locus between Oosterhoff types I and II in the Bailey diagram, we argue that M15 is indeed a bona-fide Oosterhoff type II