Astrophysical Quantities of Cepheid Variables Measured with the Navy Prototype Optical Interferometer

We present mean angular diameters for two Cepheid variables, α UMi and ζ Gem, determined with the Navy Prototype Optical Interferometer (NPOI). We present linear radii for these Cepheids and two additional Cepheids, δ Cep and η Aql, previously observed at the NPOI. We find the limb-darkened angular diameters of α UMi and of ζ Gem to be 3.28 ± 0.02 and 1.55 ± 0.09 mas, respectively. Using trigonometric parallaxes, we find the linear radii of α UMi, ζ Gem, δ Cep, and η Aql to be 46 ± 3, 60 , 45 , and 69 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. We find that the observed diameter of α UMi is in excellent agreement with the predicted diameter as determined from both surface brightness techniques and theory only if α UMi is a first-overtone pulsator.