  We have found an optical/X-ray counterpart candidate for the bright, but
presently unidentified, Fermi source 2FGL J1311.7-3429. This counterpart
undergoes large amplitude quasi-sinusoidal optical modulation with a 1.56h
(5626s) period. The modulated flux is blue at peak, with T_eff ~14,000K, and
redder at minimum. Superimposed on this variation are dramatic optical flares.
Archival X-ray data suggest modest binary modulation, but no eclipse. With the
gamma-ray properties, this appears to be another black-widow-type millisecond
pulsar. If confirmation pulses can be found in the GeV data, this binary will
have the shortest orbital period of any known spin-powered pulsar. The flares
may be magnetic events on the rapidly rotating companion or shocks in the
companion-stripping wind. While this may be a radio-quiet millisecond pulsar,
we show that such objects are a small subset of the gamma-ray pulsar
population.
