  RX J1856.5-3754, a bright soft X-ray source believed to be the nearest
thermally emitting neutron star, has commanded and continues to command intense
interest from X-ray missions. One of the main goals is to determine the radius
of this neutron star. An integral part of the determination is an accurate
parallax. Walter (2001) analyzed Hubble Space Telescope (HST) data and derived
a parallax, pi=16.5+/-2.3 mas. Combining this distance with the angular radius
derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT,
EUVE, HST, Pons et al. (2001) derived an observed radius ("radiation radius"),
R_{infty}= 7 km. This value is smaller than the radii calculated from all
proposed equations-of-state (EOS) of dense baryonic matter (Haensel 2001).
Here, we have analyzed the same HST data and find pi=7+/-2 mas. We have
verified our result using a number of different, independent techniques, and
find the result to be robust. Adopting our parallax, the radius of RX
J1856.5-3754 is R_{infty}=15+/-6 km. This radius falls squarely in the range of
radii, 12--16 km, expected from calculations of neutron star structure for
different equations of state. With additional HST observations, the parallax
estimate can be improved to the point by which the inferred radius can
constrain the choice of EOS.
