The stabilization of the 3 hour period of Nova Cygni 1975

Photometry of V1500 Cyg through October 1978 establishes that the period of its 3-hour variation stabilized at 0.139617 + or - 0.000002 day in early 1977. The amplitude of the variation changes from cycle to cycle, but in 1978 averaged about 0.65 mag in the continuum. Rapid flickering is also present at all phases of the 3-hour cycle. Changes in the period and amplitude of the variation are consistent with the Fabian-Pringle (1977) model, in which the periodicity during 1975-1976 arises from a spiral density variation in an outflowing wind. The observed coronal emission lines may be the spectral signature of the stellar wind. This model requires continuing mass loss for about 500 days after outburst and a total mass ejected of 0.001 solar mass. The mechanism for the constant-period light variations in 1977-1978 is probably different, and a simple HZ Her-like model is discussed.