The outer-halo globular cluster NGC 6229. IV. Variable stars

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Abstract
We present time-series B, V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the Borissova et al. (1997) candidate list located very close to the cluster center. Two variable stars suspected by Carney, Fullton, & Trammell (1991) are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates from Borissova et al. (1997) are confirmed variables -- three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one eclipsing binary (most likely an Algol system) and one bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard (1999) has been employed which, together with the Welch & Stetson (1993) technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRab's, whereas the other two are RRc's. As originally suggested by Mayer (1961), NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are compared against those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.

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