  We have obtained snapshot images of a transient X-ray source in M31 from
Chandra ACIS-I and the Hubble Space Telescope (HST) Advanced Camera for Surveys
(ACS). The Chandra position of the X-ray nova was R.A.=00:42:56.038 +/-0.08'',
Dec.=+41:12:18.50 +/-0.07''. The transient was active for at least 6 months.
Previous observations set an upper limit before the X-ray outburst,
demonstrating variability by a factor of >100 and confirming the transient
nature of the source. For the first 6 months after the initial detection, the
X-ray luminosity was ~6 X 10$^{37}$ erg/s; it then decayed to <5 X 10$^{36}$
erg/s over the following 2 months. An HST observation 29 days after the initial
X-ray detection revealed a source at R.A.=00:42:56.042, Dec.=+41:12:18.45 that
was B=24.52 +/- 0.07. This optical source faded to B=24.95 +/- 0.08$ in 9
months. The HST identification of an optical source at the same position as the
X-ray source, fading in concert with the X-ray source, indicates that this
optical source is the counterpart of the X-ray transient. The lack of high-mass
stars in the region suggests this source is a low-mass X-ray binary, and the
X-ray and optical luminosities provide a rough orbital period estimate of
8$^{+12}_{-5}$ days for the system.
