Search for evolutionary changes in Cepheid periods using the Harvard plate collection: RS Puppis

Based on photographic plates from the Harvard College Observatory, we have made 1492 magnitude estimates for the long-period classical Cepheid RS Pup (P = 410.3em{·}d 4) . Together with the observations taken from the literature, our data have allowed us to construct an O-C diagram spanning a time interval of 135 years. The O-C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary increase in the period, P = 119.3 (±3.0) s yr−1, in agreement with the results of theoretical calculations for the third crossing of the instability strip. The available data reduced by the method of Eddington and Plakidis (1929) have revealed random fluctuations in the period of ∼ 00.3em{·}d 225 . However, they do not dominate on the O-C diagram, because the progressive evolutionary changes in the period in a 135-year interval lead to an amplitude of the evolutionary changes in the O-C residuals that exceeds the amplitude of the random fluctuations in the O-C residuals by a factor of 3.