Resonances among the pulsation modes in Cepheids give rise to observable effects on the light curves which can be exploited to put constraints on the pulsational models and on the mass-luminosity relations. The best known of these resonances occurs in the fundamental mode Cepheids between the fundamental and the second overtone mode () in the vicinity of a period d and it is at the origin of the well known Hertzsprung progression of the bump Cepheids (e.g. Simon and Lee 1981). In the first overtone mode Cepheids another resonance occurs between the first and the fourth pulsation modes (; e.g. Antonello & Poretti 1986; Antonello, Poretti and Reduzzi 1990). When the resonance effects observed in light curves of Cepheids of Galaxy and Magellanic Clouds are used to constrain purely radiative models, one obtains stellar masses that are too small to be in agreement with stellar evolution calculations (e.g. Buchler 1998). Moreover, the radiative models predict a sensitivity of the Fourier parameters of light curves to the metal content Z, for P close to the resonance center , but this sensitivity is not confirmed by Cepheids in Magellanic Clouds (low metallicity galaxies). According to Buchler et al. (1999), it has become clear that some form of convective transport and of turbulent dissipation is needed to make progress. However, even this new treatment does not appear to be sufficient for reproducing correctly the main observational features (J.R. Buchler, private communication; G. Bono, private communication). We are therefore compelled to supply theorists with new observational data, in particular of Cepheids in very metal poor irregular galaxies such as IC 1613, to help them in the attempt to solve the problems.
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000