Search for Evolutionary Changes in the Periods of the Cepheids CEa Cas and CEb Cas

For two classical Cepheids, CEa Cas and CEb Cas, members of the open cluster NGC 7790, we have constructed $$O-C$$ diagrams spanning a time interval of 125 years. The $$O-C$$ diagrams have the shape of parabolas, which has allowed us for the first time to determine the quadratic light elements and to calculate the rates of evolutionary changes in their periods: $$dP/dt={-}0.038({\pm}{0.017})$$ s yr $${}^{-1}$$ for CEa Cas and $$dP/dt={+}0.099({\pm}{0.010})$$ s yr $${}^{-1}$$ for CEb Cas, in agreement with the theoretical calculations for the second and third crossings of the instability strip, respectively. The pulsation stability test proposed by Lombard and Koen has confirmed that the changes in the periods are real.