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It is now 75 years since the usefulness of classical Cepheid variables for distance determination was first recognized by Henrietta Leavitt. Since that time, the distances to which Cepheids have been sought has increased from 60 kpc to 15-20 Mpc. Our ability to understand the factors affecting Cepheid luminosities and their calibration has also grown, especially in recent years. In this paper, I will provide a brief introduction to Cepheid pulsation and evolution, and then describe in more detail the recent observational results obtained from the field and cluster Cepheid populations of the LMC and SMC. The influence of metallicity and age on the filling of the instability strip will be clearly demonstrated.
As the distance to a population of Cepheid variables is increased, our ability to extract information about individual stars is reduced. Multicolor photometry and abundance analyses are possible for LMC and SMC Cepheids. Within the Local Group, BVRI photometry is possible, and so a period-luminosity-color relation may still be employed. At the greatest distances, only one- or two-color photometry may be obtained. The effect of this loss of information and the expedients used to offset it will be described.
Of particular importance is the influence of metallicity on the luminosity of Cepheids, since the local calibrators, the LMC and SMC are both metal-poor compared to field Galactic Cepheids and some of the extragalactic samples. The recent metallicity study of M31 by Freedman and Madore (1991) gives hope that any dependence of luminosity on metallicity is small, but has been criticized by Feast (1991) as being incompatibile with the known effects of line-blanketing on stellar colors. Possible explanations and avenues for resolving these disparate claims will be discussed.
Finally, our experience in searching for Cepheid and long-period variables in the Virgo cluster spiral NGC 4571 with the Canada- France-Hawaii Telescope and High-Resolution Camera will be described and the unique problems of data reduction which appear when studying galaxies at distances of 15-20 Mpc will be discussed. I will report on the progress of this survey after two full observing seasons.
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