Superhumps in Cataclysmic Binaries. V. 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 nova. 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.