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F. M. Walter (SUNY Stony Brook)
Direct measurements of the masses and radii of neutron stars promise to constrain the equation of state of nuclear matter. Masses are best measured in binary systems, but the cleanest measurements of radii are obtained from isolated single neutron stars. The isolated compact object RX J185635-3754 has a thermal spectrum, so atmospheric models will return an angular diameter. Together with the measured distance, this fixes the radius. The broadband spectral energy distribution from ROSAT, EUVE, and the HST shows that the surface composition is not hydrogen, but could be heavy elements (Fe or Si-ash). I will review the observations and the results of non-magnetic atmospheric model fitting. The resulting radii are small, and inconsistent with many published equations of state. I will discuss the implications of these results, and will anticipate what we may learn from high quality X-ray spectra of this and other isolated neutron stars.