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±2.05 s yr^‑1, corresponding to log(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.