SIMBAD references

321 Galactic fundamental-mode Cepheids with good B, V, and (in most cases) I photometry by Berdnikov et al. (2000A&AS..143..211B) and with homogenized color excesses E(B-V) based on Fernie et al. (1995IBVS.4148....1F) are used to determine their period-color (P-C) relation in the range 0.4<logP≲1.6. The agreement with colors from different model calculations is good to poor. - Distances of 25 Cepheids in open clusters and associations (Feast, 1999PASP..111..775F) and of 28 Cepheids with Baade-Becker-Wesselink (BBW) distances (Gieren et al., 1998ApJ...496...17G) are used in a first step to determine the absorption coefficients R_B=4.17, R_V=3.17, and R_I=1.89 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 0.12mag ±0.04mag . They are therefore combined into a single mean Galactic P-L_B,V,I relation. The analysis of HIPPARCOS parallaxes by Groenewegen & Oudmaijer (2000A&A...356..849G) gives absolute magnitudes which are brighter by 0.21mag ±0.11mag in V and 0.18mag ±0.12mag in I at logP=0.85. Agreement with P-L relations from different model calculations for the case [Fe/H]=0 is impressive to poor. Galactic Cepheids are redder in (B-V)⁰ than those in LMC and SMC as shown by the over 1000 Cloud Cepheids with good standard B, V, I photometry by Udalski et al. (1999AcA....49..223U, 1999AcA....49..437U); the effect is less pronounced in (V-I)⁰. Also the (B-V)⁰, (V-I)⁰ 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 ΔlogT_e∼0.02 at M_V=-4.0mag, 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 0.5mag at short periods (logP=0.4) and fainter at long periods (logP>1.4). The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at logP=1.0 towards still shallower values as shown in a forthcoming paper.