The Cepheids V371 Per and OGLE-LMC-CEP-2132: The First Crossing of the Instability Strip

For both periods of the double-mode Cepheid V371 Per and for the Cepheid OGLE-LMC-CEP-2132 we have constructed \(O-C\) diagrams spanning a time interval of 126 and 119 years, respectively. 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: \(dP_{Fu}/dt=1.085({\pm}{0.007})\) s yr\({}^{-1}\) and \(dP_{1O}/dt=0.923({\pm}{0.003})\) s yr\({}^{-1}\) for the fundamental mode and the first overtone of V371 Per, respectively, and \(dP/dt=15.304({\pm}{0.048})\) s yr\({}^{-1}\) for OGLE-LMC-CEP-2132, in agreement with the results of theoretical calculations for the first crossing of the instability strip. The pulsation stability test proposed by Lombard and Koen has confirmed that the increase in the periods is real.