We will compare the light curves of Cepheids in IC 1613 with those in Galaxy. First of all, as remarked in Paper II, we note that the shape of the light curves in the Wh band does not differ significantly from that in the V band, since the effective wavelengths of the two passbands are close; moreover, as regards the observed amplitudes, . According to the study of BVRI light curves of galactic Cepheids made by Simon & Moffett (1985), the values are independent of the filter, while the values increases with the wavelength. values for the V band differ by about 0.2-0.3 rad from those for R band. Since Wh effective wavelength is intermediate between V and R ones, we expect that the values of Wh light curves are systematically larger by 0.1 rad than those of V light curves, which is a negligible quantity in the economy of the present comparative study.
The Fourier parameters are plotted against P in Fig. 1. The values for d tend to be larger in IC 1613; this feature should depend on the metallicity since it is also noted in the Small Magellanic Cloud Cepheids (see Paper II). The values decrease with P, but unfortunately there are no many stars, and hence we cannot identify precisely a minimum at d as in Galaxy. The values for d appear to be essentially the same in both galaxies. For longer periods, the values do not change very much from star to star in IC 1613, and it is possible that, differently from the Galaxy, at 10 d the values are rather uniform, i.e. there is no spread.
Differences between the stars with P near 8 - 9 d in the two galaxies are seen also for values. In order to understand this point, we have compared the sequence of the curves of the Cepheids in the two galaxies for the limited period range near 8 - 9 d; the curves are plotted in Fig. 2.
Some typical examples of light curves of galactic Cepheids are shown, taken from the paper by Antonello & Morelli (1996). The most evident feature is a double maximum; for d the first (in time) maximum is higher than the second, while for either the opposite occurs, or the light curve is featureless (or at most with a small bump on the ascending branch, e.g. FN Aql). Note that the almost sinusoidal light curves have very small values, but the amplitude is not negligible. This is generally verified by all the galactic Cepheids with P between 8 and 10 d (Fig. 1). In IC 1613 we find a star, V1274-B, with 8.48 d, whose light curve shape is similar to that of galactic Cepheids with d, while the stars V1638-C and V1193-C have light curve shapes similar to those of galactic Cepheids with d. Moreover it is interesting to note that the amplitudes of the Wh light curves of V1274-B, V1638-C and V1193-C are larger than those of V light curves of galactic Cepheids with similar P.
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000