We present mean angular diameters for two cepheid variables, α Ursae Minoris and ζ Geminorum, determined with the Navy Prototype Optical Interferometer (NPOI). We present linear radii for these cepheids and two additional cepheids, δ Cephei and η Aquilae, previously observed at the NPOI. We find the limb-darkened angular diameter of α Ursae Minoris and of ζ Geminorum to be 3.28 ± 0.02 and 1.55 ± 0.09 milliarcseconds respectively. Using trigonometric parallaxes, we find the linear radii of α Ursae Minoris, ζ Geminorum, δ Cephei and η Aquilae to be 46 (± 3) R⊙, 60 (+25, -14) R⊙, 45 (+8, -6) R⊙, and 69 (+28, -15) R⊙ respectively. We compare the pulsation periods and linear radii of this sample of cepheids, which range in period from three 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 Ursae Minoris is in excellent agreement with the predicted diameter as determined from both surface brightness techniques and theory only if α Ursae Minoris is a first overtone pulsator. Subject headings: Cepheids — stars: fundamental parameters — stars: individual (Polaris, ζ Geminorum, δ Cephei, η Aquilae)

The proposed design and construction of the Fizeau astrometric mapping explorer (FAME), a small astrometric instrument for use on an artificial satellite, is reported on. The instrument and spacecraft are designed to slowly spin and will repeatedly scan great circles on the sky so that, over a period of time, it will cover the complete sky and repeat in a manner similar to that of the Hipparcos satellite. The instrument will use the two fixed dilute aperture telescopes to measure a fixed angle between stars and detect the positions, magnitude and color of all stars crossing its field of view to a visual magnitude of approximately 15 mag. The aim of the instrument is to obtain a catalog of positions, proper motions and parallaxes of all stars down to about 15 mag, with a magnitude dependent accuracy of positions of 20 to 800 micro arcsec, proper motions of 20 to 800 micro arcsec per year and parallaxes of 20 to 800 micro arcsec.

We have measured the diameters of the Cepheid variablesCephei (18 nights) and � Aquilae (11 nights) with the Navy Prototype Optical Interferometer. The primary results of these observations are the mean angular diameters hLDi of these Cepheids: 1.520 ± 0.014 milliseconds of arc (mas) forCep and 1.69 ± 0.04 mas forAql. We also report limb-darkened diameters for the check stars in this program: forLac, �LD = 1.909 ± 0.011 mas, and for 12 Aql, �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� for both Cepheids.