Cepheids as Distance Indicators

We review the use of Cepheids as distance indicators with particular emphasis on the methods which have been applied to HST observations of Cepheids. The calibration of the period-luminosity relations is examined in detail and we identify possible problems with the existing calibrations. New V- and I-band period luminosity relations are presented based on a sample of 53 Cepheids in the LMC with photometry drawn from the literature. These revised PL relations result in a systematic decrease of approx 0.1 magnitudes in distance moduli derived using the standard method of extinction correction. Hence estimates of the Hubble constant based on such distances should be increased by $\sim5%$. Other aspects of Cepheid distance determination, specifically incompleteness bias, metallicity dependence, the effects of crowding and contamination of samples by non-Cepheids are also discussed. We conclude that current HST distance estimates to individual galaxies are probably good to about 10%, but that much of this error is systematic. Efforts to reduce the systematics, therefore, for example by improving the photometric calibration, refining the distance to the LMC, and reobserving the Cepheid galaxies in the infrared with NICMOS, will give large returns.