BOOST OF THE ORBITAL MOTION IN HIGH-MASS X-RAY BINARIES

We investigate the orbital behaviour of close high mass X-ray binaries (HMXBs), including the effects of the evolutionary growth of the mome nt of inertia and mass loss by stellar wind from the main-sequence sta r. We find that a nearly synchronous state implies in the context of t idal evolution a rapid decrease of the orbital period for binary syste ms close to the Darwin instability. In this frame, the observed orbita l decays \P/P\greater than or equal to 10(-6) yr(-1) in the Cen X-3 an d SMC X-1 systems appear to be characteristic of synchronous HMXBs con taining a neutron star. The rate of change in orbital period is expect ed to be one or two orders of magnitude smaller in close HMXBs with bl ack hole candidates.