We present simultaneous high-resolution optical spectroscopy and X-ray data of the X-ray binary system GR Mus ( XB 1254–690 ) , obtained over a full range of orbital phases .
The X-ray observations are used to re-establish the orbital ephemeris for this source .
The optical data includes the first spectroscopic detection of the donor star in this system , through the use of the Doppler Tomography technique on the Bowen fluorescence blend ( \sim 4630-4650 Å ) .
In combination with an estimate for the orbital parameters of the compact object using the wings of the He ii \lambda 4686 emission line , dynamical mass constraints of 1.20 \leq M _ { X } / M _ { \sun } \leq 2.64 for the neutron star and 0.45 \leq M _ { 2 } / M _ { \sun } \leq 0.85 for the companion are derived .