UBVRI photometry of W Ursae Majoris

High speed UBVRI photometry of W Ursae Majoris produced 15,930 observations covering one complete cycle on 1984 March 5-6. For this epoch, total phase of primary minimum is flat in all spectral bands, while secondary minimum shows a substantial tilt during annual phases. The tilt correlates with a detectable O'Connell effect, indicates a transverse temperature gradient across the projected disk of the primary component, and is qualitatively consistent with a starspot origin for both effects. Gross characteristics of the color curves are consistent with those expected from the wavelength dependence of limb darkening. The observed greater color change at primary minimum as contrasted with secondary must have a different origin. There is qualitative consistency with a Rucinski hot secondary model. Whether consistency can also be achieved with a Mullan starspot model, or whether decisive discrimination between alterative physical models can be demonstrated must await detailed light synthesis simulation.