2001AJ....121..476A

We have measured the diameters of the Cepheid variables δ Cephei (18 nights) and η Aquilae (11 nights) with the Navy Prototype Optical Interferometer. The primary results of these observations are the mean angular diameters <θ_LD> of these Cepheids: 1.520±0.014 milliseconds of arc (mas) for δ Cep and 1.69±0.04 mas for η Aql. We also report limb-darkened diameters for the check stars in this program: for β Lac, θ_LD=1.909±0.011 mas and for 12 Aql, θ_LD=2.418±0.010 mas. When combined with radius estimates from period-radius relations in the literature, the Cepheid angular diameters suggest distances slightly smaller than, but still consistent with, the Hipparcos distances. Pulsations are weakly detected at a level of ∼1.5 to 2 σ for both Cepheids.