A NEW MASS ESTIMATE FOR HERCULES-X-1

New optical spectroscopy of the 1.7-d eclipsing binary pulsar HZ Her/H er X-l is presented. The data are of higher quality than those used in previous studies. After making appropriate corrections for the distor tion in the velocity curve due to the strong X-ray heating, the masses of the binary components are derived. The neutron star is found to ha ve a mass of 1.5 +/- 0.3 M., in contrast to the last radial velocity s tudy which found a value of less than 1.0 M.. Rotational velocity meas urements, while insufficiently precise to reject the older mass estima te, suggest that the rotational period of HZ Her differs from the sync hronous rate by no more than 20 per cent. Using a recently determined figure for the effective temperature of the unheated face, the distanc e to Her X-l is estimated at 6.6 +/- 0.4 kpc, higher than prior estima tes. The systemic velocity is found to be -65 +/- 2 km s(-1).