Superhumps in cataclysmic binaries. XVI. DI Ursae Majoris

We report photometry of the dwarf nova DI Ursae Majoris during 1997-1998. T he star shows an orbital light curve at quiescence with a period of 0.05456 4(1) days. During superoutburst, it flashes superhumps with a period of 0.0 5529(5) days. The superhump period excess of 1.3% is quite low among catacl ysmic variables, suggesting a weak perturbation on the accretion disk and t herefore a secondary of very low mass. But other stars of low period excess are rare eruptors, whereas DI UMa is extremely active with normal outburst s every 8 days and superoutbursts every 30-45 days. This is puzzling. We sp eculate that the binary really does have a secondary of anomalously low mas s but is presently in a high state of a mass-transfer cycle.