Astrophysical quantities of Cepheid variables measured with the navy prototype optical interferometer

We present mean angular diameters for two Cepheid variables, alpha UMi and zeta Gem, determined with the Navy Prototype Optical Interferometer (NPOI). We present linear radii for these Cepheids and two additional Cepheids, delta Cep and eta Aql, previously observed at the NPOI. We find the limb-darkened angular diameters of UMi and of zeta Gem to be 3.28 +/- 0.02 and 1.55 +/- 0.09 mas, respectively. Using trigonometric parallaxes, we find the linear radii of alpha UMi, zeta Gem, delta Cep, and eta Aql to be 46 =/- 3, 60(-14)(+25), 45(-6)(+28), and 69(-15)(+28) R., respectively. We compare the pulsation periods and linear radii of this sample of Cepheids, which range in period from 3 to 11 days, to theoretical and empirical period-radius and period-radius-mass relations found in the literature. Ne find that the observed diameter of a UMi is in excellent agreement with the predicted diameter as determined from both surface brightness techniques and theory only if a UMi is a first-overtone pulsator.