COMPARISON OF THE OPEN CLUSTER AND SURFACE-BRIGHTNESS DISTANCE SCALES FOR GALACTIC CLASSICAL CEPHEIDS

We have derived a new period-luminosity (PL) relation for galactic Cepheids in open clusters and associations from the Zero-Age Main-Sequence (ZAMS)-fitting method, using the most reliable cluster photometric data and Cepheid reddening data available, and homogeneous treatment of absorption corrections. For about half of the cluster Cepheid sample (17 stars) we also derive distances and absolute magnitudes from the surface-brightness method. From a comparison we find that the surface-brightness PL relation has a zero point about 0.15 mag brighter than the cluster ZAMS-fitting PL relation, but in view of the sources of systematic error involved in both methods, this discrepancy is clearly not significant. Rather, we conclude from our results that the two currently most important methods to derive distances to galactic Cepheids produce consistent results, and that neither of the two methods contain systematic errors in excess of approximately 0. 10 mag. The results of this paper imply a distance to the Large Magellanic Cloud of mu0 (LMC) = 18.65 +/- 0.10 mag. We identify four stars in our cluster/association Cepheid sample for which the discrepancies between ZAMS-fitting and surface brightness M(V) are so large ( > 0.6 mag) that serious doubt is cast on their membership to the proposed host clusters or associations. These cases deserve further study.