ИСТИНА

The principal objective of our work is to search for evolutionary period changes in the most rapidly evolving Cepheids: small-amplitude Cepheids (DCEPS) and long-period Cepheids (P>30 days) (DCEP). We used the Hertzsprung method to determine the times of brightness maxima. Our analysis of the O-C diagrams showed that evolutionary period changes (if any) are virtually always accompanied by wavelike cyclic oscillations of O-C residuals (with time scales of 10-20-30 years) that are sometimes superimposed by cyclic oscillations of smaller amplitudes and periods. The latter, in turn, often consist of straight- line segments (SZ Cas, X Lac, Y Oph, GY Sge, S Vul, SV Vul =AE ET Vul). In cases when the amplitude of the O-C residual oscillations mentioned above is small compared to the progressive period change, the parameters of the parabola defining the evolutionary part of period changes can be determined with much confidence. Otherwise, the identification of evolutionary period changes in a Cepheid becomes a very difficult task (e.g., IR Cep, X Lac, and ET Vul).