  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.
