SUPERHUMPS IN CATACLYSMIC BINARIES .5. V503 CYGNI

We report a radial-velocity study of the dwarf nova V503 Cygni at quiescence, revealing motion of the emission lines with a period of 111.9+/-0.4 min. This is almost certainly the underlying orbital period of the binary. We also report photometry at all stages of the outburst cycle, which demonstrates a complex pattern of signals. In superoutburst, the star shows a periodic signal at 116.7 +/- 0.1 min, which bears all the earmarks of a ''superhump,'' a normal trait of SU UMa-type dwarf novae. During quiescence and short eruptions, the star shows a large-amplitude wave at a period of 109.00+/-0.02 min-a ''negative superhump'' since the period shift from P(orb) is negative. We interpret the two superhumps as, respectively, arising from apsidal and nodal precession of the accretion disk.