We present a phase-resolved, optical, spectroscopic study of the eclipsing
low-mass X-ray binary, EXO 0748-676 = UY Vol. The sensitivity of Gemini
combined with our complete phase coverage makes for the most detailed blue
spectroscopic study of this source obtained during its extended twenty-four
year period of activity. We identify 12 optical emission lines and present
trailed spectra, tomograms, and the first modulation maps of this source in
outburst. The strongest line emission originates downstream of the
stream-impact point, and this component is quite variable from night-to-night.
Underlying this is weaker, more stable axisymmetric emission from the accretion
disk. We identify weak, sharp emission components moving in phase with the
donor star, from which we measure Kem = 329+/-26 km/s. Combining all the
available dynamical constraints on the motion of the donor star with our
observed accretion disk velocities we favor a neutron star mass close to
canonical (M1~1.5Msun) and a very low mass donor (M2~0.1$Msun). We note that
there is no evidence for CNO processing that is often associated with
undermassive donor stars, however. A main sequence donor would require both a
neutron star more massive than 2Msun and substantially sub-Keplerian disk
emission.Comment: Accepted for publication in the Astrophysical Journa
