Optical spectroscopy of the massive X-ray binary SMC X-1/Sk 160.

New spectroscopy of Sk 160, the optical counterpart of SMC X-1, is presented. Improved mass estimates are derived for both components of the binary system, with and without non-Keplerian velocity corrections. The primary star is found to have a mass of 17.2 +/- 0.6 M_sun_, with a best estimate for the mass of the compact component of 1.61+/- 0.1 M_sun_, which is within the theoretical mass range for neutron stars. Studies of line profile variations in the primary show that the effect of X-ray heating on the inner hemisphere is present but not dramatic. Distance estimates are derived, the lowest of which implies a sub-Eddington luminosity for the X-ray source, and places the system on the near side of the SMC.