  We propose to determine the mass and the radius of a neutron star (NS) using
three measurable mass-radius relationships, namely the ``apparent'' radius
inferred from neutron star thermal emission, the gravitational redshift
inferred from the absorption lines, as well as the averaged stellar mass
density inferred from the orbital Keplerian frequency derived from the
kilohertz quasi periodic oscillation (kHz QPO) data. We apply the method to
constrain the NS mass and the radius of the X-ray sources, 1E 1207.4-5209, Aql
X-1 and EXO 0748-676.
