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Astron. Astrophys. 363, 601-604 (2000)

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1. Introduction

As a by-product of microlensing surveys (MACHO, EROS and OGLE projects), huge amounts of data on light curves of Magellanic Cloud Cepheids with short period P are now available, which allows significant tests of nonlinear model predictions. In a comparison between Galactic and Magellanic Cepheid light curves, Buchler et al. (1996) pointed out a serious problem for stars in a period range around 10 days, while the advent of the OPAL opacity reduces the beat Cepheid problems (e.g. Morgan & Welch 1997). The theoretical models are not able to reproduce the observed features when going from the metal content of Galaxy ([FORMULA]) to that of LMC ([FORMULA]) and SMC ([FORMULA] 0.005; the metal content for the Magellanic Cloud Cepheids has been estimated spectroscopically e.g. by Luck et al. 1998). A similar conclusion to that of Buchler et al. (1996) was obtained by Wood et al. (1997) after the comparison of the observed light curve of the Cepheid HV 905 with nonlinear pulsation models. The authors claimed that in order to get a good agreement with the observed light curve, a quite `luminous' mass-luminosity M-L relation should be supposed. Owing to this, the overshooting which would be required in the evolution calculations is roughly the double of the currently used values. In the present paper, we report about the results of a study of nonlinear models for a wide range of pulsation periods, obtained for two M-L relations: one is derived from stellar evolution calculations with fully convective overshooting (e.g. Chiosi 1990), and the other is that suggested by Wood et al. (1997). The light and velocity curves of the limit cycles are compared with observations in terms of the Fourier components; the available data taken from the literature were analyzed by Antonello (1998). Carson & Stothers (1984) pointed out that the models of very long period Cepheids do not follow the usual linear period-luminosity PL relation, and this result was used to explain the observed behavior of the relation for [FORMULA] d. In the present paper we will further detail this behavior; moreover, we will also briefly discuss some nonlinear effects related to resonances.

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000