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Investigating period changes of classical Cepheids through the framework of$O-C$ diagrams provides a unique insight to the evolution and nature of thesevariable stars. In this work, the new or extended $O-C$ diagrams for 148Galactic classical Cepheids are presented. By correlating the calculated periodchange rates with the Gaia EDR3 colours, we obtain observational indicationsfor the non-negligible dependence of the period change rate on the horizontalposition within the instability strip. We find period fluctuations in 59Cepheids with a confidence level of 99%, which are distributed uniformly overthe inspected period range. Correlating the fluctuation amplitude with thepulsation period yields a clear dependence, similar to the one valid for longerperiod pulsating variable stars. The non-negligible amount of Cepheids showingchanges in their $O-C$ diagrams that are not or not only of evolutionary originpoints toward the need for further studies for the complete understanding ofthese effects. One such peculiar behaviour is the large amplitude periodfluctuation in short period Cepheids, which occurs in a significant fraction ofthe investigated stars. The period dependence of the fluctuation strength andits minimum at the bump Cepheid region suggests a stability enhancing mechanismfor this period range, which agrees with current pulsation models.

Study of changes in the pulsation period of 148 Galactic Cepheid variables