2000A&A...363..601A

Nonlinear, radiative models for long-period Cepheids in the Magellanic Clouds were constructed and the theoretical light and velocity curves were compared with observations. We assumed a metal content Z=0.005, and two mass-luminosity M-L relations: one was obtained from stellar evolution calculation with convective overshooting models, and the other, which gives an even larger L for the same M, was suggested by the study of a bump Cepheid in the Magellanic Clouds. Both relations produce results which are in rough agreement with the observed light and radial velocity curves of Cepheids with period P>30d. The latter relation, however, gives better results when compared with observations for P∼10d. The longest P of stable limit cycle is about 130d; in a previous paper we obtained, for Z=0.020, a maximum P of about 70d. This result compares reasonably well with Cepheids observed in Galaxy and Magellanic Clouds; in other words, the longest P of Cepheids should depend on the average metallicity of the parent galaxy. The nonlinear P of more massive models with low metallicity tends to lengthen significantly for a very small increase of M, and this could explain the flattening of the PL relation for P>100d, which is observed in metal poor galaxies. A discussion of the shortcomings of the models for a period as short as about 10d and some comments on resonance effects are also reported.