  We consider the precession rates of eccentric discs in close binaries, and
compare theoretical predictions with the results of numerical disc simulations
and with observed superhump periods. A simple dynamical model for precession is
found to be inadequate. For mass ratios less than approximately 1/4 a linear
dynamical model does provide an upper limit for disc precession rates. Theory
suggests that pressure forces have a significant retrograde impact upon the
precession rate (Lubow 1992). We find that the disc precession rates for three
systems with accurately known mass ratios are significantly slower than
predicted by the dynamical theory, and we attribute the difference to pressure
forces. By assuming that pressure forces of similar magnitude occur in all
superhumping systems, we obtain an improved fit to superhump observations.
