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Period Variations of Cepheids and Stellar Evolution

Classical Cepheids are pulsating supergiants in the main instability strip. In the course of evolution, they move across theinstability strip, and their periods vary. Studying their period variations for a time span of several decades, it is possible todetermine the rate of secular period increase or decrease, identify the number of instability strip crossing, and thus improvethe period–luminosity relation. Special tests are used to determine whether the observed period variation is real or causedby random errors.