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 M-2/M-1 = 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 = 50degrees +/- 5degrees, M-1 = 0.63(-0.09)(+0.12) M-circle dot, M-2 = 0.12(-0.02)(+0.03) M-circle dot, R-2 = 0.121(-0.007)(+0.010) R-circle dot, and a distance to the binary of 180 +/- 40 pc.