HST imaging of MEGA microlensing candidates in M31

We investigate HST/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernovae are possible for two). If these lenses arise in M31's halo, they are due to masses 0.15 <m/M. <0.49 (95% certainty, for a delta-function mass distribution), with brown dwarfs for disk lenses and stellar masses 0.49 for bulge lenses.