  The transient X-ray source CXOM31 004205.77+411330.43 exhibited several
outbursts during our long-term monitoring campaign of ~monthly observations of
the M31 center with Chandra. However, the decay profile appears to be unlike
those observed from Galactic transients. We followed up the 2011 outburst with
two ~B band HST/ACS observations, one in outburst and the other in quiescence,
and used difference imaging to search for a counterpart; this would be
dominated by re-processed X-ray emission from the disk. We found a counterpart
with B = 28.21+/-0.16. An XMM-Newton observation from a previous outburst
yielded a spectrum that is well described by an absorbed power law with
absorption equivalent to ~2.6 E+21 H atom cm^(-2) and photon index ~1.8; the
highest quality Chandra spectrum yielded only $\sim$130 counts, and best fits
consistent with the XMM-Newton results. We calculated an absolute V magnitude
of +1.9 during outburst for a typical disk spectrum. An empirical relation
between the ratio of X-ray to optical flux and orbital period suggests a period
< ~4 hr for a black hole accretor. Such a short period is expected to yield an
asymmetric, precessing disk, and we propose that the observed decay lightcurve
is due to modulation of the mass transfer rate due to the disk precession.
