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

We present time-series B, V photometry of NGC 6229, obtained with the purpo se 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, wit h the exception of five stars from the candidate list of Borissova et al. l ocated very close to the cluster centre. Two stars suspected to be variable s by Carney, Fullton and Trammell are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates of Borissova ct al. are confirmed variab les - three of them being fundamental RR Lyrae (RRab) pulsators, two first- overtone pulsators, one an eclipsing binary (most likely an Algol system) a nd one a bright star whose variability status could not be satisfactorily d etermined. A new image subtraction method (ISM) suggested by Alard has been employed which, together with the Welch-Stetson technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemer ides are provided. Ten of these are RRabs, whereas the other two are RRcs. As originally suggested by Mayer, 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 ph ysical properties of the NGC 6229 RR Lyrae population are contrasted with t hose of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.