:WE; have succeeded in constructing the theoretical formulas for the masses, gravitational redshifts, and radii of low-mass neutron stars as functions of the star's central density and the new EOS parameter
ηη
in a manner that is consistent with empirical masses and radii of stable nuclei. The value of
ηη
, which characterizes the stiffness of neutron star matter, remains unknown, but could be deduced from possible simultaneous
MM
and
RR
measurements via comparison with our formulas if the star observed is light enough. Thus, firm evidence for the presence of low-mass neutron stars is first of all desired. One promising candidate is the neutron star in the high-mass X-ray binary 4U 1538-52, of which the mass could be significantly low or even the lowest among stars with known mass if the binary orbit is eccentric [37,38]. The X-ray burster 4U 1724-307 in the globular cluster Terzan 2 is even more interesting because the X-ray data from the cooling phase of photospheric radius expansion bursts apparently allow the object to have a relatively low mass and still a significantly large radius [39]. Such conclusions are tentative partly because of the dependence on the atmosphere models adopted and partly because of uncertainties in the distance to the object, but, if valid, might eventually suggest the
ηη
value of the order of, or even larger than, 130 MeV.
