M4 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 M4. The input data are a large and unprecedented set of deep Hubble Space Telescope WFC3 images (large program GO-12911; 120 orbits allocated), primarily aimed at probing binaries with massive companions by detecting their astrometric wobbles. Though these data were not optimised 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spatial resolution and dynamic range enabled us to firmly detect 38 variable stars, of which 20 were previously unpublished.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 cluster-member eclipsing binaries (confirming the large binary fraction of M4), RR Lyrae, and objects with known X-ray counterparts. We improved and revised the parameters of some among published variables. 
V.Nascimbeni, L. R. Bedin, D. C. Heggie, M. van den Berg, M. Giersz,   G. Piotto, K. Brogaard, A. Bellini, A. P. Milone, R. ...
astro-ph.SR astro-ph.EP astro-ph.IM