Phase Lag of Classical Cepheids and RR Lyrae Stars

It is a well-known property of Cepheids and RR Lyrae variables that their maximum brightness approximately coincides with the maximum expansion velocity. A more detailed investigation (Carson & Stothers 1984; Simon 1984) has shown that the velocity curve is in fact slightly retarded with the respect to the light variations. In this report we discuss the trends in the observed phase lag for an extensive sample of classical Cepheids and RRab variables.