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We report the detection, with HST, of an optical counterpart to the transient supersoft X-ray source 1E 1339.8+2837, in the globular cluster M3. The counterpart is found near the faint end of the subgiant branch in the V vs V-I color magnitude diagram, but is extremely bright in U. Variability is detected over a range of timescales suggesting the presence of an accretion disk and perhaps also ellipsoidal variations of the subgiant secondary. The optical colors of the binary are similar to those of cataclysmic variables recently discovered in 47 Tucanae and NGC 6397. We suggest that magnetically channeled accretion may explain the relatively low X-ray luminosity of this source's supersoft state., Comment: 11 pages, 3 figures, 2 tables. Accepted for publication in ApJ

The 1991 ROSAT PSPC M31 X-ray point source catalog has been screened in order to set up a sample of candidate supersoft sources in this galaxy, additional to the 16 supersoft sources of Supper et al. (1997). 26 candidates have been found. Absorbing hydrogen column densities, effective temperatures and white dwarf masses (assuming the sources are on the stability line of surface nuclear burning) are derived. An observed white dwarf mass distribution is derived which indicates that the masses are constrained to values above 0.90 solar masses. The entire population of supersoft sources in M31 is estimated taking a theoretical white dwarf mass distribution into account, assuming that the observationally derived sample is restricted to white dwarf masses above 0.90 solar, and taking into account that the gas and the source population have a different scale height. A total number of at least 200-500 and at most 6,000-15,000 sources is deduced depending on the used galaxy N_H model, making use of the population synthesis calculation of Yungelson (1996). The spatial distribution favors a disk (or spiral-arm) dominated young stellar population with a ratio of 1/(4-7) of bulge/disk systems. Assuming that all supersoft sources with masses in excess of 0.5 solar masses are progenitors of supernovae of type Ia, a SN Ia rate of (0.8-7) 10^-3 yr^-1 is derived for M31 based on these progenitors. Supersoft sources might account for 20-100% of the total SN Ia rate in a galaxy like M31., Comment: 13 pages, accepted by A&A January 11th 1999