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Astron. Astrophys. 349, 55-69 (1999)

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1. Introduction

1.1. Motivation

Cepheids are variable stars which are used to measure distances of galaxies in the Local Group and nearby clusters (e.g. Madore et al. 1998), and are the primary calibrator for the secondary standard candles that are applied at much greater distances (e.g. Jacoby et al. 1992). However, they are not only fundamental stars as primary distance indicators, but are also an essential tool for testing the theories on the internal constitution of stars and stellar evolution. The importance of double-mode Cepheids for the revision of stellar opacities is well known: after the suggestion of Simon (1982), the OPAL (e.g. Iglesias et al. 1990) and OP (e.g. Seaton et al. 1994) projects produced new opacities which allowed to solve the long-standing problem of double-mode Cepheid period ratios (e.g. Moskalik et al. 1992). These opacities were then generally adopted by theorists working with stellar evolution codes.

There are several problems yet to be solved. The radiative codes used for constructing pulsation models proved to be incapable of agreement with observations when applied to the comparison of Cepheid characteristics in Galaxy and in Magellanic Clouds (e.g. Buchler 1998). The fact that resonances among the pulsation modes give rise to observable effects on the light curves can be exploited to put constraints on the pulsational models and on the mass-luminosity relations. The best known of these resonances occurs in the fundamental Cepheids between the fundamental and the second overtone mode ([FORMULA]) in the vicinity of a period [FORMULA] d and it is at the origin of the well known Hertzsprung progression of the bump Cepheids (e.g. Simon & Lee 1981). In the first overtone mode Cepheids another resonance occurs between the first and the fourth pulsation modes ([FORMULA]; e.g. Antonello & Poretti 1986; Antonello et al. 1990). When these resonances observed in Cepheids of Galaxy and Magellanic Clouds are used to constrain purely radiative models, one obtains stellar masses that are too small to be in agreement with stellar evolution calculations. According to Buchler et al. (1999), it has become clear that some form of convective transport and of turbulent dissipation is needed to make progress.

The study of Cepheids in nearby galaxies is of fundamental importance for understanding the effects of different metallicity and corresponding mass-luminosity relations on the pulsational characteristics through the detection of structures in the Fourier parameter - period diagrams of fundamental, first overtone and possibly double-mode (Poretti & Pardo 1997) and second overtone mode Cepheids (Antonello & Kanbur 1997; Alcock et al. 1999), and their comparison with the galactic Cepheids and the model predictions. The CCD differential photometric precision allows to get accurate Fourier parameters of Cepheid light curves, and also to discover several new Cepheids with small amplitude. Massive CCD photometry of nearby galaxies such as NGC 6822 and IC 1613 was attempted several years ago by E. Schmidt and collaborators (Schmidt & Spear 1989), but apart from a preliminary report, no complete study was published. The MACHO, EROS and OGLE projects dedicated to the detection of microlensing events in the direction of Magellanic Clouds produced enormous amount of data on variable stars in these galaxies (e.g. Welch et al. 1997; Beaulieu & Sasselov 1997; Udalski et al. 1999). More recently, the project DIRECT was dedicated to the massive CCD photometry of M31 (and M33) with the purpose of detecting Cepheid and eclipsing binaries for direct distance determination of these galaxies (e.g. Kaluzny et al. 1999).

The purpose of our project was to obtain good light curves of Cepheids for extending the comparison of the characteristics of these stars in different galaxies. In order to exploit the telescope time and reach the faintest luminosities, our strategy was to observe in white light, i.e. without filter; the results confirm that in this way the differential photometry precision for the Ducth 0.91 cm telescope at ESO-La Silla is roughly comparable with that obtained with 2 m-class telescopes, Johnson V-filter and similar exposure times.

In the present work we discuss observations and reduction methods, and we present the first results concerning population I Cepheids and other variable stars. Subsequent papers will be dedicated to the analysis of population I and II Cepheids, long period and irregular variables and eclipsing binaries.

1.2. IC 1613

The irregular galaxy IC 1613 [[FORMULA] (1950), [FORMULA] (1950), l=130o, b=-61o], was studied by Baade, but his extensive results were never published. Baade found 59 variables in plates taken with the Mount Wilson 60 inch and 100 inch reflectors between 1929 and 1937. Light curves for 24 of the confirmed Cepheids had been completed by him before his death in 1960. These data, reduced to a new photometric scale, were published by Sandage (1971), who discussed the apparently anomalous slope of the PL relation. The cause of this shallower slope than that of Cepheids in other galaxies was interpreted differently by various authors; the reasons for such interest was that if the slope was significantly flatter for IC 1613, then the assumption that a universal PL relation exists was seriously called into question. Freedman (1988a) discussed the case with new CCD BVRI data, and noted that, for data fainter than 21 mag, the photographic photometry was significantly brighter than CCD data. This divergence contributed to the difference in the appearance of the PL relation. Sandage (1988) discussed another cause, that is the stochastic effect of small sample statistics for the few longest-period variables. An additional 16 Cepheids were subsequently presented by Carlson & Sandage (1990), and the authors remarked the possible large number of short period Cepheids. The conclusion of these studies is that there are no differences in the slope of the PL relation of Cepheids in IC 1613 with respect to that of other galaxies.

From BVRI observations (Freedman, 1988a), Madore & Freedman (1991) derived a total mean reddening of E(B-V)=0.02 mag, and a true distance modulus of 24.42[FORMULA]0.13 mag, corresponding to a distance of 765 kpc. Madore & Freedman (1991) suggest that the best place for work on intrinsic calibration problem of the Cepheid distance scale is not the Magellanic Cloud system but IC 1613, because the foreground reddening to this galaxy is very low and probably quite uniform, the extinction internal to IC 1613 appears to be quite small and the crowding of stellar images are relatively low. Freedman (1988a) mentions other points of interest of this galaxy, and the need of better data on its Cepheids. IC 1613 has very low metallicity, less than SMC, and both are important galaxies for calibrating the PL relation, but the latter has a complicated extended geometry; therefore the former could be even more important in this regard than was once thought. With more and better data, the Cepheids of IC 1613 could provide the low-metallicity anchor point for a calibration of the PL relation.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999