Superhumps in Cataclysmic Binaries. XXII. 1RXS J232953.9+062814

We report photometry of a recently discovered dwarf nova with a remarkably short 64.2 minute orbital period. In quiescence, the star's light curve is that of a double sinusoid, arising from the ``ellipsoidal'' distortion of the Roche-lobe-filling secondary. During superoutburst, common superhumps develop with a period 3%-4% longer than P_orb. This indicates a mass ratio M₂/M₁=0.19+/-0.02, a surprisingly large value in so compact a binary. This implies that the secondary star has a density 2-3 times higher than that of other short-period dwarf novae, suggesting a secondary enriched by H-burning prior to the common-envelope phase of evolution. We estimate i=50^deg+/-5^deg, M₁=0.63^+0.12_-0.09 M_solar, M₂=0.12^+0.03_-0.02 M_solar, R₂=0.121^+0.010_-0.007 R_solar, and a distance to the binary of 180+/-40 pc.