Evidence for subsynchronous rotation in HZ Herculis/Hercules X-1

An analysis of the orbit of Hercules X-1 is carried out, including results from recent X-ray and optical studies. Constraints on the binary parameters are given. Since HZ Herculis is required to be inside or filling its criti cal Roche lobe, no orbital solutions are allowed unless HZ Her's rotation r ate is subsynchronous. This is consistent with the observed orbital period derivative. The alternative possibility is that the optical velocity of HZ Her is still not well determined. Assuming that it is correct, new upper an d lower limits on the neutron star mass are derived: 1.23 M. < M-x < 1.48 M ..