We obtained new optical observations of the X-ray source XMMU J083850.38 - 282756.8 , the previously proposed counterpart of the \gamma -ray source 3FGL J0838.8 - 2829 .
Time-series photometry in the r ^ { \prime } band reveals periodic modulation of \approx 1 magnitude that is characteristic of the heating of the photosphere of a low-mass companion star by a compact object .
The measured orbital period is 5.14817 \pm 0.00012 hr .
The shape of the light curve is variable , evidently due to the effects of flaring and asymmetric heating .
Spectroscopy reveals a companion of type M1 or later , having a radial velocity amplitude of 315 \pm 17 km s ^ { -1 } , with period and phasing consistent with the heating interpretation .
The mass function of the compact object is 0.69 \pm 0.11 M _ { \odot } , which allows a neutron star in a high-inclination orbit .
Variable , broad H \alpha emission is seen , which is probably associated with a wind from the companion .
These properties , as well as the X-ray and \gamma -ray luminosities at the inferred distance of < 1.7 kpc , are consistent with a redback millisecond pulsar in its non-accreting state .
A search for radio pulsations is needed to confirm this interpretation and derive complete system parameters for modeling , although absorption by the ionized wind could hinder such detection .