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We report new radial velocity observations of GP Vel/HD77581, the opticalcompanion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more thantwo complete orbits of the system, we detect evidence for tidally inducednon-radial oscillations on the surface of GP Vel, apparent as peaks in thepower spectrum of the residuals to the radial velocity curve fit. By removingthe effect of these oscillations (to first order) and binning the radialvelocities, we have determined the semi-amplitude of the radial velocity curveof GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measuredsemi-amplitude of the pulsar's orbit, the mass ratio of the system is0.081+/-0.005. We are able to set upper and lower limits on the masses of thecomponent stars as follows. Assuming GP Vel fills its Roche lobe then theinclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain themasses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star andM_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle isi=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe isbeta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun andM_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits isalso possible, corresponding to other combinations of i and beta. In addition,we note that if the zero phase of the radial velocity curve is allowed as afree parameter, rather than constrained by the X-ray ephemeris, a significantlyimproved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shiftof 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of theradial velocity curve may indicate the presence of an additional radialvelocity component at the orbital period.Comment: Accepted for publication in Astronomy & Astrophysic

The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel