A Composite Period-Luminosity Relation for Cepheids at Mean and Maximum Light

Modern two-color data for Cepheids in the Large and Small Magellanic Clouds, M31, and NGC 6822 are combined into composite period-luminosity relations in MB and M~ at mean and maximum light. The data are consistent with the view that the P-L relations are the same from galaxy to galaxy in both slope and zero point. The P-L relations show dispersions of ± 0.60 mag in MB and ±0.50 mag in M~ about the center line. The magnitude residuals, RB and R~, from the center line for individual stars, correlate well with the color residuals, ?(B - V), from the ridge line of the period-color relation. Such a correlation is ex- pected from the theory of the Cepheid instability strip in the color-magnitude diagram. Theory and ob- servation agree well, and this is taken as a proof that the observed dispersion in the P-L relation is pri- marily intrinsic. The absolute calibrations of the P-L relations are made using nine Cepheids in the galactic system whose absolute luminosities are known by photometric parallax methods Tables of the adopted period- luminosity and period-color relations are given in the Appendix