FAINT ULTRAVIOLET OBJECTS IN THE CORE OF M13 - OPTICAL COUNTERPARTS OF THE LOW-LUMINOSITY X-RAY SOURCE

The core of the Galactic globular cluster M13 (NGC 6205) has been obse rved with WFPC2 on the Hubble Space Telescope through visual, blue, an d mid-and Ear-UV filters in a programme devoted to studying the UV pop ulation in a sample of Galactic globular clusters. In the UV colour-ma gnitude diagrams derived from the HST images we have discovered three faint objects with a strong UV excess, which lie significantly outside the main loci defined by more than 12 000 normal cluster stars. The p ositions of two of the UV stars are nearly coincident (7 and 1 arcsec) with those of a low-luminosity X-ray source recently found in the cor e of M13 and with a 3.5-sigma peak in the X-ray contour map. We sugges t that the UV stars are physically connected to the X-ray emission. Th e UV stars are very similar to the quiescent nova in the globular clus ter M80, and they might be a, perhaps new, subclass of cataclysmic var iable.