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

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.