We thank Dr J. Bailey for making available his computer light curves;
Drs J. E. Pringle;
J;
A;
Whelan;
A. C. Fabian and A. Schwarzenberg-Czerny;
for useful discussions. We gratefully acknowledge the help of D. A. Findley in translating data tapes. M. C. Cook acknowledges the support of an SERC research studentship during the performance of this work;
which is now supported by NSF grant No PHY80-25605;