A search for evolutionary changes in the pulsation period of the classical Cepheid V609 Cygni

Abstract

A total of 820 magnitude estimates were made for the long-period classical Cepheid V609 Cyg ($P =31.1$ days) using the photographic plate collections of the Harvard College Observatory and Sternberg Astronomical Institute. Combined with published photometry, the data cover a time baseline of about 129 years on the resulting $O$–$C$ diagram. That circumstance allowed us to detect an evolutionary increase in the pulsation period at a rate of $21.38 \pm 2.05$ s yr⁻¹, corresponding to $\log (\dot{P} /P) = -5.10$, which agrees well with theoretical predictions for a star in the third crossing of the instability strip. When reduced using the method introduced by Eddington and Plakidis (Mon. Not. R. Astron. Soc. 90:65, 1929) the available data demonstrate the presence of mild random period fluctuations, which do not mask the evolutionary changes in the $O$–$C$ residuals.