Warped accretion discs and the long periods in X-ray binaries

Precessing accretion discs have long been suggested as explanations for the long periods observed in a variety of X-ray binaries, most notably Her X-1 /HZ Her. We show that an instability of the response of the disc to the rad iation reaction force from the illumination by the central source can cause the disc to tilt out of the orbital plane and precess in something like th e required manner. The rate of precession and disc tilt obtained for realis tic values of system parameters compare favourably with the known body of d ata on X-ray binaries with long periods. We explore other possible types of behaviour than steadily precessing discs that might be observable in syste ms with somewhat different parameters. At high luminosities, the inner disc tilts through more than 90 degrees, i.e., it rotates counter to the usual direction, which may explain the torque reversals in systems such as 4U 162 6-67.