New Period-Luminosity and Period-Color relations of classical Cepheids: I. Cepheids in the Galaxy

321 Galactic fundamental-mode Cepheids with good B, V, and (in most cases) I photometry by Berdnikov et al. (CITE) and with homogenized color excesses based on Fernie et al. (CITE) are used to determine their period-color (P-C) relation in the range . The agreement with colors from different model calculations is good to poor. – Distances of 25 Cepheids in open clusters and associations (Feast CITE) and of 28 Cepheids with Baade-Becker-Wesselink (BBW) distances (Gieren et al. CITE) are used in a first step to determine the absorption coefficients , , and appropriate for Cepheids of intermediate color. The two sets of Galactic Cepheids with known distances define two independent P-L relations which agree very well in slope; their zero points agree to within . They are therefore combined into a single mean Galactic P-L relation. The analysis of HIPPARCOS parallaxes by Groenewegen the effect is less pronounced in . Also the , two-color diagrams differ between Cepheids in the Galaxy and the Clouds, attributed both to the effects of metallicity differences on the spectral energy distributions of the Cepheids and to a shift in the effective temperature of the middle of the instability strip for LMC and SMC relative to the Galaxy by about at , hotter for both LMC and SMC. Differences in the period-color relations between the Galaxy and the Clouds show that there cannot be a universal P-L relation from galaxy-to-galaxy in any given color. The inferred non-uniqueness of the slope of the P-L relations in the Galaxy, LMC, and SMC is born out by the observations. The Cloud Cepheids follow a shallower overall slope of the P-L relations in B, V, and I than the Galactic ones. LMC and SMC Cepheids are brighter in V than in the Galaxy by up to at short periods () and fainter at long periods (). The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at towards still shallower values as shown in a forthcoming paper.