Fundamental data for contact binaries : RZ Com, RZ Tau and AW UMa.

Differential corrections analyses of three binaries of the W UMa type show that RZ Tau and AW UMa have common envelopes and the relatively small gravity darkening predicted by Lucy, while RZ Com seems to have a larger gravity effect and is only marginally in contact. Some details of our method for computing contact-binary light curves are given. We suggest that W UMa binar- ies which have occultation primary eclipses may, in general, have large amplitudes for gravity darkening or a photometric surface-brightness effect which simulates large gravity darkening. Subject headings: eclipsing binaries - W Ursae Majoris stars