A COMPARISON BETWEEN OBSERVATIONS AND NONLINEAR MODELS OF FIRST OVERTONE MODE CEPHEIDS

Light and radial velocity curves of observed first overtone mode Cephe ids have been compared with the curves of nonlinear models constructed using the new opacities. The study confirms that the characteristics of the curves depend on the resonance P-1/P-4 = 2 between the first an d the fourth overtone mode. However, discrepancies and systematic diff erences between theory and observations are still present. It is shown that the diagrams of amplitude ratios R(21) and phase difference phi( 21) of light curves versus the period are sensitive to the T-e of the models. Interesting properties of the phase lag between light and radi al velocity curve are also discussed.