Phase Lags and Pulsation Modes of Classical Cepheids

The first-order phase lag between light and velocity is calculated from Fourier decompositions of the variations of 12 classical Cepheids. The phase lags have a narrow range, with maximum expansion velocity always retarded with respect to maximum light. The average value of the first-order phase lag is found to be approximately -0.3, close to the theoretical value obtained from the models of Simon and Davis. When the phase lag is plotted against period, two of the stars stand out. These stars are SU Cas and AZ Cen, both suspected overtone pulsators. We further investigate AZ Cen by combining the light curves of three observers and plotting the star on the cP21-period diagram of Simon and Lee. The tentative result is seen to strengthen both the identification of AZ Cen as an overtone pulsator and the possible role of the first-order phase lag as a discriminator of mode. We close with a discussion of our results and of the uncertainties which remain. Subject headings: stars: Cepheids stars: pulsation