  We discuss efforts to determine the mass, radius, and surface composition of
the nearby compact object RX J185635-3754 from its multi-wavelength spectral
energy distribution. We compute non-magnetized model atmospheres and emergent
spectra for selected compositions and gravities, and discuss efforts to fit
existing and new observational data from ROSAT, EUVE and the HST. The spectral
energy distribution matches that expected from a heavy-element dominated
atmosphere, but not from a uniform temperature blackbody. Non-magnetic light
element atmospheres cannot be simultaneously reconciled with the optical and
X-ray data. We extend previous studies, which were limited to one fixed neutron
star mass and radius. For uniform temperature models dominated by heavy
elements, the redshift z is constrained to be 0.3 < z < 0.4 and the best-fit
mass and radius are M approx 0.9 solar masses and R approx 6 km (for a 61 pc
distance). These values for M and R together are not permitted for any
plausible equation of state, including that of a self-bound strange quark star.
A simplified two-temperature model allows masses and radii up to about 50%
larger, or a factor of 2 in the case of a black body. The observed luminosity
is consistent with the thermal emission of an isolated neutron star no older
than about 1 million years, the age inferred from available proper motion and
parallax information
