  The discovery of photospheric absorption lines in XMM-Newton spectra of the
X-ray bursting neutron star in EXO0748-676 by Cottam and collaborators allows
us to constrain the neutron star mass-radius ratio from the measured
gravitational redshift. A radius of R=9-12km for a plausible mass range of
M=1.4-1.8Msun was derived by these authors. It has been claimed that the
absorption features stem from gravitationally redshifted (z=0.35) n=2-3 lines
of H- and He-like iron. We investigate this identification and search for
alternatives. We compute LTE and non-LTE neutron-star model atmospheres and
detailed synthetic spectra for a wide range of effective temperatures
(effective temperatures of 1 - 20MK) and different chemical compositions.
  We are unable to confirm the identification of the absorption features in the
X-ray spectrum of EXO0748-676 as n=2-3 lines of H- and He-like iron (Fe XXVI
and Fe XXV). These are subordinate lines that are predicted by our models to be
too weak at any effective temperature. It is more likely that the strongest
feature is from the n=2-3 resonance transition in Fe XXIV with a redshift of
z=0.24. Adopting this value yields a larger neutron star radius, namely
R=12-15km for the mass range M=1.4-1.8Msun, favoring a stiff equation-of-state
and excluding mass-radius relations based on exotic matter. Combined with an
estimate of the stellar radius R>12.5km from the work of Oezel and
collaborators, the z=0.24 value provides a minimum neutron-star mass of
M>1.48Msun, instead of M>1.9Msun, when assuming z=0.35.
