Search for Evolutionary Changes in the Periods of Cepheids: BY Cas

Abstract

For the low-amplitude Cepheid BY Cas we have constructed an O - C diagram spanning a time interval 123 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, dP/dt = +0.985 (±0.091) s yr^-1, which points to the third crossing of the instability strip. The pulsation stability test proposed by Lombard and Koen (1993) confirms that the period increase is real.