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We have measured the diameters of the Cepheid variables delta Cephei (18nights) and eta Aquilae (11 nights) with the Navy Prototype OpticalInterferometer. The primary results of these observations are the mean angulardiameters <theta(LD)> of these Cepheids: 1.520 +/- 0.014 milliseconds of arc(mas) for delta Cep and 1.69 +/- 0.04 mas for eta Aql. We also reportlimb-darkened diameters for the check stars in this program: for beta Lac,theta(LD) = 1.909 +/- 0.011 mas, and for 12 Aql, theta(LD) = 2.418 +/- 0.010mas. When combined with radius estimates from period-radius relations in theliterature, the Cepheid angular diameters suggest distances slightly smallerthan, but still consistent with, the Hipparcos distances. Pulsations are weaklydetected at a level of about 1.5 sigma to 2 sigma for both Cepheids.Comment: 15 pages, including one figure; LaTeX 2e; uses aastex, aas_symbols,  natbib; accepted for publication in the Astronomical Journa

Diameters of δ Cephei and η Aquilae Measured with the Navy Prototype Optical Interferometer