The distance scale for classical Cepheid variables

New radii, derived from a modified version of the Baade-Wesselink (BW) method that is tied to published KHG narrowband spectrophotometry, are presented for 13 bright Cepheids. The data yield a best-fitting period-radius relation given by log [R/R.] = 1.071(+/-0.025) + 0.747(+/-0.028) log P(0). In combination with other high-quality radius estimates recently published by Laney & Stobie, the new data yield a period-radius relation described by log [R/R.] = 1.064(+/-0.0006) + 0.750(+/-0.006) log P(0), which simplifies to [R] infinity P(3/4). The relationship is used to test the scale of Cepheid luminosities inferred from cluster zero-age main-sequence (ZAMS) fitting, for which we present an updated list of calibrating Cepheids located in stellar groups. The cluster ZAMS-fitting distance scale tied to a Pleiades distance modulus of 5.56 is found to agree closely with the distance scale defined by Hipparcos parallaxes of cluster Cepheids and also yields Cepheid luminosities that are a good match to those inferred from the period-radius relation. The mean difference between absolute visual magnitudes based on cluster ZAMS fitting, [M(V)](C), and those inferred for 23 cluster Cepheids from radius and effective temperature estimates, [M(V)](BW),in the sense of C BW is + 0.019 +/- 0.029 s.e. There is no evidence to indicate the need for a major revision to the Cepheid cluster distance scale. The absolute magnitude differences are examined using available [Fe/H] data for the cluster Cepheid sample to test the metallicity dependence of the period-luminosity relation. Large scatter and a small range of metallicities hinder a reliable estimate of the exact relationship, although the data are fairly consistent with predictions from stellar evolutionary models. The derived dependence is DeltaM(V)(C-BW) = + 0.06(+/-0.03) - 0.43(+/-0.54)[Fe/H].