A search for evolutionary period variations of Cepheids using the harvard plate stacks: II Carinae

Abstract

We obtained 380 photographic brightness estimates for the long-period classical Cepheid II Car (P = 64.4 days) from the plate stacks of the Harvard College Observatory. Combined with the published photoelectric and CCD observations, these data enable the construction of an O-C diagram covering 120 years. The O-C diagram is parabolic, making it possible for the first time to determine the quadratic light-curve elements and calculate the evolutionary rate of increase of the period, dP/dt = 719(±15) s/year (log(Ṗ/P) = −4.64), in agreement with theoretical computations for the third crossing of the instability strip. The available data, reduced using the method described by Eddington and Plakidis, demonstrate the presence of small random period fluctuations that do not distort the evolutionary changes of the O-C residuals.