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 Ma gellanic Clouds using the visual surface brightness technique and the excel lent light curves of these variables given by Moffett et al. Combining thes e new radii with infrared radii of shorter period Galactic Cepheids, we pro vide a new calibration of the period-radius (PR) relation which covers the entire period spectrum of Cepheid variables (up to 130 days). We find evide nce that the Cepheid period-radius relation is independent of the galaxy en vironment (e.g., universal) and linear up to the largest pulsation periods. Furthermore, the intrinsic dispersion of the relation is small enough to p ermit successful use of the Cepheid PR relation in the determination of rea sonably accurate pulsation parallaxes to nearby galaxies.