Variable stars in nearby galaxies. VI. Frequency-period distribution of Cepheids in IC 1613 and other galaxies of the Local Group

The frequency-period distribution and other properties of Cepheids in IC 1613 are discussed and compared with those of stars in our Galaxy (Milky Way), LMC, SMC, M 31 and M 33. Taking into account the observational limitations and related incompleteness, it is concluded that the frequency-period distribution of Cepheids in IC 1613 is similar to that of SMC; we suspect that a much larger number of stars exist in IC 1613 with a period of less than 2 d that have not yet been detected. A discussion of the deficiency of fundamental mode Cepheids with periods in the range 8-10 d in the Milky Way, M 31 and M 33 is reported. The present data are not sufficient to verify if this is produced by a real bimodal frequency-period distribution or whether depends on the lack of pulsating stars in such a period range due to pulsational stability reasons. Some arguments are presented in favor of a bimodal distribution that is a function of the average metallicity. The Milky Way, M 31 and M 33 have the two maxima located at the same periods, about 5 and 13 d, respectively. A comment on very long period Cepheids is also given. Based on observations collected at ESO-La Silla.