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16. Comparisons with other distance indicators

As recently as a decade ago, the range of published Cepheid distances to nearby galaxies was sufficiently large that by adopting one or another calibration could result in differences in the Hubble constant of almost a factor of two. Fortunately, a decade later, distance determinations to nearby galaxies have been obtained with new CCD data using a number of independent techniques (Cepheids, RR Lyraes, TRGB, and even type II supernovae in the case of SN 1987 in the LMC). Now that photometry with linear detectors is available for a variety of methods, and corrections for reddening can be applied, the distances to nearby galaxies have converged to full range differences of less than 0.3 mag (i.e., 15% in distance, Freedman & Madore 1993). Moreover, the excellent agreement of individual distances gives no indication of large remaining systematic errors.

However, as noted by Freedman & Madore (1993), although the Cepheid and RR Lyrae distances agree to within their stated errors, the differences are systematic (in the sense that the RR Lyrae distances are smaller than the Cepheid distances). This effect has been noted and discussed by Walker (1992), Hazen & Nemec (1992) and Reid & Freedman (1994) in the case of the LMC, and Saha et al. (1992) in the case of IC 1613. Most recently, this effect has been discussed by van den Bergh (1995). As yet unresolved are the slope and zero points of the relation between absolute magnitude and the metallicity for RR Lyrae stars, as well as the metallicity sensitivity of the Cepheid PL relations as a function of wavelength. However, the fact that the Population I Cepheid distances now agree as well as they do (to within 0.15 to 0.30 mag) with the Population II RR Lyrae and TRGB distances, is consistent with the predictions of a small Cepheid PL dependence on metallicity by Chiosi et al. (1993), and the observational result of Freedman & Madore (1990).