An unusually low mass of some "neutron" stars?

The X-ray emission of RXJ1856.5-3754 has been found to coincide to unprecedented accuracy with that of a blackbody, of radius 5.8+-0.9 km for the measured parallax distance of 140 pc (Burwitz et al. 2001, Drake et al. 2002). If the emission is uniform over the whole surface of a non-rotating star, the mass of the star cannot exceed 0.75+-0.12 solar mass regardless of its composition. If the compact object is a quark star described by the MIT-bag equation of state (a ``strange star''), the mass is no more than 0.3 solar mass. Comparably small masses are also obtained for the X-ray bursters Aql X-1 and KS1731-260 for some fits to their spectra.