Calibrating the Cepheid Period‐Radius Relation with Galactic and Magellanic Cloud Cepheids

We have determined the radii of 16 long-period Cepheid variables in both Magellanic Clouds using the visual surface brightness technique and the excellent light curves of these variables given by Moffett et al. Combining these new radii with infrared radii of shorter period Galactic Cepheids, we provide a new calibration of the period-radius (PR) relation which covers the entire period spectrum of Cepheid variables (up to 130 days). We find evidence that the Cepheid period-radius relation is independent of the galaxy environment (e.g., universal) and linear up to the largest pulsation periods. Furthermore, the intrinsic dispersion of the relation is small enough to permit successful use of the Cepheid PR relation in the determination of reasonably accurate pulsation parallaxes to nearby galaxies.