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We are presently engaged in a program of observations to determine if the Galactic and M31 dark halos are composed of Massively Condensed Halo Objects (MACHOs), and, if so, their mass distribution and geometry in the halo of M31. The experiment is described in Crotts (1992) and involves imaging a large number of stars (specifically red giants) in the inner 2-3 kpc of M31. A MACHO (either in the Galactic or M31 halos) that passes in front of a background star in M31 will reveal itself through gravitational lensing which leads to an amplification of the background star's light. Similar studies are being performed by other groups monitoring stars in the LMC or Galactic Bulge to look for Galactic halo MACHOs. However, as discussed in Crotts (1992), there are a number of extremely important advantages with the M31 experiment, including a a lensing probability for M31 MACHOs that is a strong function of galactocentric position implying that that microlensing events can be proven on a {\it statistical} basis.
The stars in M31 are unresolved and individual stars cannot be photometered from the ground. However, we demonstrate that variability can be readily detected among these stars using a technique involving image subtraction. We discuss results from a four night imaging survey of the inner bulge of M31 using the KPNO 4-m telescope during September 1994. A number of detections are made of stellar variability in M31 stars. The possible identity of these detections as microlensing events from either Galactic or M31 MACHOs is analysed and presented. The implications of this survey's sensitivity to Galactic MACHO microlensing events up to three orders of magnitude lower mass range than other ongoing surveys is discussed. Initial results from a longer term program of observations started this Fall using the Mount Graham Vatican 1.8-m telescope are also presented. This survey is geared towards detecting a wide range of possible M31 MACHO masses as well as extensively surveying the variable star population of the inner bulge of M31.
