Optical eclipses and precession effects in the x-ray binaries SMC X-1 and Centaurus X-3

An analysis is made of all published B, V photometry for the x-ray binary systems SMC X-1 and Cen X-3. Both systems are eclipsing variables in the optical range. The eclipses of the visible star are caused by gas streams and the accretion structure (disk) surrounding the neutron star. The V amplitude of the ''pure'' ellipsoidal variability is roughly-equal0/sup m/.08 for SMC X-1 and roughly-equal0/sup m/.05 for Cen X-3; the total amplitude of the optical variability is roughly-equal0/sup m/.11 and roughly-equal0/sup m/.09, respectively. A substantial reflection effect is also observed in the SMC X-1 system. Long-period (precessional) variability in both shape and amplitude of the optical light curves is established with periods P1 = 35/sup d/.25 for SMC X-1 and P1 = 26/sup d/.1 for Cen X-3. In Cen X-3 a 26/sup d/.1 cycle is also detected in the intensity variations of the x-ray dips, indirectly indicating that the optical-star rotation axis is precessing. The long-period variability is induced by changes in the orientation of the eclipsing gas streams and accretion structure; in the case of SMC X-1 long-term variations also result from the changing conditions whereby the powerful x-ray emission of the accreting neutron star heats the optical star. These effects evince a forced precession of the optical-star rotation axis, which may have been tilted away from the normal to the orbit plane by the supernova explosion in the system