The precession of eccentric discs in close binaries

We consider the precession rates of eccentric discs in close binaries, and compare theoretical predictions with the results of numerical disc simulati ons and with observed superhump periods. A simple dynamical model for prece ssion is found to be inadequate. For mass ratios less than or similar to 1/ 4 a linear dynamical model does provide an upper limit for disc precession rates. Theory suggests that pressure forces have a significant retrograde i mpact upon the precession rate. We find that the disc precession rates for three systems with accurately known mass ratios are significantly slower th an predicted by the dynamical theory, and we attribute the difference to pr essure forces. By assuming that pressure forces of similar magnitude occur in all superhumping systems, we obtain an improved fit to superhump observa tions.