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Astron. Astrophys. 333, L35-L38 (1998)

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1. Introduction

The longer period Cepheids are particularly important in the context of the primary distance scale because they are bright enough to be visibile at a great distance. However, due to their low number, they have been poorly studied both observationally and theoretically. Simon & Kanbur (1995) considered 50 Cepheids in Galaxy and IC 4182 with period P less than 70 d, compared them with hydrodynamical pulsation models and concluded that a detailed comparison between theory and observations must await a more extensive and accurate sample of observed stars. Antonello & Morelli (1996) analyzed all the available photometric V data of galactic Cepheids with period less than 70 d looking for possible resonance effects; they noted some small features in the Fourier parameters-period diagrams which were ascribed tentatively to expected resonances. Aikawa & Antonello (1997) tried to reproduce these observations with nonlinear models, but their conclusion was that the increasing nonadiabaticity of the pulsation with period probably reduces the effectiveness of resonance mechanisms. Finally, Simon & Young (1997) studied long period Cepheids in the period range [FORMULA] d in Magellanic Clouds looking for galaxy-to-galaxy differences in the Cepheid distributions. Resonances between pulsation modes, which were studied essentially in Cepheids with P less than about 30 d, represent a powerful comparison tool between observations and theoretical model predictions, because they affect the shape of the curves of pulsating stars in specific period ranges. The comparison of the Fourier parameters of observed and theoretical light and radial velocity curves allows to probe the stellar interior and to put constraints on the stellar physical parameters. After the work of Simon & Lee (1981) on the resonance [FORMULA] at [FORMULA] d between the fundamental and second overtone mode in classical bump Cepheids, several papers by various authors were devoted to this topic, from both the observational and theoretical point of view. For example, Buchler & Kovacs (1986) and Moskalik & Buchler (1989) studied the general effects of 2:1 and 3:1 resonances in radial stellar pulsations and discussed the possible astrophysical implications, Petersen (1989) discussed the possible two- and three-mode resonances in Cepheids, and Antonello (1994) looked for the expected effects in short period Cepheids. Recent reviews on galactic and Magellanic Cloud Cepheids pulsating in fundamental and first overtone mode and on the problems raised by the comparison with the pulsational models are those by Buchler (1996) and Beaulieu & Sasselov (1996). The resonance effects in Magellanic Cloud Cepheids cannot be reproduced by models constructed using current input physics and reasonable mass-luminosity relations; in particular the case of first overtone Cepheids characterized by [FORMULA] (Antonello et al. 1990) has proven to be rather difficult for theorists. We mention in passing also the recent resonance [FORMULA], studied in the models of hypothetical second overtone mode Cepheids (Antonello & Kanbur 1997).

In the present work we have considered the long-period Cepheids in Magellanic Clouds, with available photometric and radial velocity data which were suitable for Fourier decomposition. The initial purpose of the work was simply to extend the comparison between theory and observations to Cepheids with the longest known periods, but the probable discovery of a new resonance effect suggested to publish the present Letter in advance of the comparison with the hydrodynamical models (Antonello & Aikawa, in preparation).

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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