We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748 - 676 after the source returned to quiescence in the fall of 2008 .
The X-ray light curve displays eclipses consistent in orbital period , orbital phase and duration with the predictions and measurements before the return to quiescence .
H \alpha and He I emission lines are present in the optical spectra and show the signature of the orbit of the binary companion , placing a lower limit on the radial velocity semi-amplitude of K _ { 2 } > 405 km s ^ { -1 } .
Both the flux in the continuum and the emission lines show orbital modulations , indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star .
Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion ; in fact no stellar absorption lines are seen in the spectrum .
Nevertheless , our observations place a stringent lower limit on the neutron star mass of M _ { 1 } > 1.27 M _ { \odot } .
For the canonical neutron star mass of M _ { 1 } = 1.4 M _ { \odot } , the mass ratio is constrained to 0.075 < q < 0.105 .