THE GLOBULAR CLUSTER NGC 5286. II. VARIABLE STARS

We present the results of a search for variable stars in the globular cluster (GC) NGC 5286, which has recently been suggested to be associated with the Canis Major dwarf spheroidal galaxy. Fifty-seven variable stars were detected, only 19 of which had previously been known. Among our detections one finds 52 RR Lyrae (22 RRc and 30 RRab), 4 long-period variables, and 1 type II Cepheid of the BL Herculis type. Periods are derived for all of the RR Lyrae as well as the Cepheid, and BV light curves are provided for all the variables. The mean period of the RRab variables is < Pab > = 0.656 days, and the number fraction of RRc stars is N(c)/N(RR) = 0.42, both consistent with an Oosterhoff Pi (Oo Pi) type-thus making NGC 5286 one of the most metal-rich ([Fe/H] = -1.67) OoII globulars known to date. The minimum period of the RRabs, namely P(ab,min) = 0.513 d, while still consistent with an OoII classification, falls toward the short end of the observed P(ab,min) distribution for OoII GCs. As was recently found in the case of the prototypical OoII GC M15 ( NGC 7078), the distribution of stars in the Bailey diagram does not strictly conform to the previously reported locus for OoII stars. We provide Fourier decomposition parameters for all of the RR Lyrae stars detected in our survey, and discuss the physical parameters derived therefrom. The values derived for the RRcs are not consistent with those typically found for OoII clusters, which may be due to the cluster's relatively high metallicity-the latter being confirmed by our Fourier analysis of the ab-type RR Lyrae light curves. Using the recent recalibration of the RR Lyrae luminosity scale by Catelan & Cortes, we derive for the cluster a revised distance modulus of (m - M)(V) = 16.04 mag.