SX phoenicis stars in the globular cluster M 13

Abstract

We present results of the search for SX Phe stars in the globular cluster M 13. In photometric reductions we used the image subtraction method supplemented with procedure of searching for short-period variable stars, based on the examination of each pixel in the CCD frames. We illustrate also the ability of this approach to find variability with small signal-to-noise ratio. Using this method we were able to derive the light curves for all three SX Phe stars known in the cluster. Moreover, we discovered another star of this type. Except v37, all detected SX Phe stars have full-amplitudes in V smaller than 0.25 mag. Combining our ground-based data and the HST observations we determined the average V magnitudes for these stars. One SX Phe star, v46, shows double-mode pulsations which are very likely the radial fundamental mode and the first overtone.