A search for evolutionary period variations of Cepheids using the Harvard plates: V396 Cyg

We obtained 503 photographic brightness estimates for the long-period classical Cepheid V396 Cyg (P = 33.3 days) from the plates of the Harvard Observatory and the Sternberg Astronomical Institute (SAI). Combined with published photoelectric observations, these data were used to plot an O-C diagram covering a time interval of 108 years. The O-C diagram is parabolic in shape, enabling the first determination of quadratic light elements and calculation of the evolutionary rate of decrease of the period, dP/dt = 14.1(±0.8) s per year (log(?/P) = ?6.35), in agreement with theoretical computations for the second crossing of the instability strip. The available data, reduced using the method introduced by Eddington and Plakidis, demonstrate the presence of small random period fluctuations that do not distort the evolutionary changes of the O-C residuals.