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

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4. Conclusion

The analysis of the radial velocity and light curves of the long-period Cepheids in the Magellanic Clouds indicates the presence of a progression which we interpret tentatively as an effect of the resonance [FORMULA] between the fundamental and the first overtone mode. Since the longest period Cepheids are also the brightest, the present results could be of some importance for the study of stellar structure and evolution in far galaxies, because the stars with [FORMULA] 100 d ([FORMULA] mag) are about three magnitudes brighter than those at 10 d, in which the well known resonance [FORMULA] is occurring. The disadvantage is the low number of such stars. Few Cepheids with [FORMULA] d have been found in relatively nearby galaxies (NGC 6822, IC 1613, NGC 300; see e.g. Madore 1985), while in the Magellanic Clouds there are just a few of stars in comparison with a total of some thousand Cepheids. Presently the Hubble Space Telescope Key Project on the Extragalactic Distance Scale is optimized for the detection of Cepheids with period between 3 and 60 d (e.g. Ferrarese et al. 1996), therefore it is not possible to derive reliable conclusions about the number of long-period stars. We just note that in NGC 925 Silbermann et al. (1996) found 4 stars with probable [FORMULA] d over a total of 80 Cepheids.

Assuming that a sufficient number of such stars is detected and our interpretation is correct, the comparison of the observed resonance effect with nonlinear model predictions will allow to test the input physics and put constraints on the physical parameters of the stars in relatively far galaxies in the same way as it is occurring for the Galaxy and Magellanic Cloud Cepheids with shorter periods.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998