The M 4 Core Project with HST - III. Search for variable stars in the primary field

We present the results of a photometric search for variable stars in the core of the Galactic globular cluster Messier 4 (M 4). The input data are a large and unprecedented set of deep Hubble Space Telescope WFC3 images (large programme GO-12911; 120 orbits allocated), primarily aimed at probing binaries with massive companions by detecting their astrometric wobbles. Though these data were not optimized to carry out a time-resolved photometric survey, their exquisite precision, spatial resolution and dynamic range enabled us to firmly detect 38 variable stars, of which 20 were previously unpublished. They include 19 clustermember eclipsing binaries (confirming the large binary fraction ofM4), RR Lyrae and objects with known X-ray counterparts. We improved and revised the parameters of some among published variables.