  The mass and radius of the neutron star (NS) in low-mass X-ray binaries can
be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass
and radius constrains the properties of dense matter in NS cores. A critical
ingredient for spectral fits is the composition of the NS atmosphere: hydrogen
atmospheres are assumed in most prior work, but helium atmospheres are possible
if the donor star is a helium white dwarf. Here we perform spectral fits to
XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13.
This NS has the smallest inferred radius from previous spectral fitting.
Assuming an atmosphere composed of hydrogen, we find a significantly larger
radius, more consistent with those from other quiescent NSs. With a helium
atmosphere (an equally acceptable fit), we find even larger values for the
radius.
