  We report the discovery of an eclipsing low-mass X-ray binary at the center
of the 3FGL error ellipse of the unassociated Fermi/Large Area Telescope
gamma-ray source 3FGL J0427.9-6704. Photometry from OGLE and the SMARTS 1.3-m
telescope and spectroscopy from the SOAR telescope have allowed us to classify
the system as an eclipsing low-mass X-ray binary (P = 8.8 hr) with a main
sequence donor and a neutron star accretor. Broad double-peaked H and He
emission lines suggest the ongoing presence of an accretion disk. Remarkably,
the system shows shows separate sets of absorption lines associated with the
accretion disk and the secondary, and we use their radial velocities to find
evidence for a massive (~ 1.8-1.9 M_sun) neutron star primary. In addition to a
total X-ray eclipse of duration ~ 2200 s observed with NuSTAR, the X-ray light
curve also shows properties similar to those observed among known transitional
millisecond pulsars: short-term variability, a hard power-law spectrum (photon
index ~ 1.7), and a comparable 0.5-10 keV luminosity (~ 2.4 x 10^33 erg/s). We
find tentative evidence for a partial (~ 60%) gamma-ray eclipse at the same
phase as the X-ray eclipse, suggesting the gamma-ray emission may not be
confined to the immediate region of the compact object. The favorable
inclination of this binary is promising for future efforts to determine the
origin of gamma-rays among accreting neutron stars.
