  The line profiles from rotating neutron stars are affected by a number of
relativistic processes such as Doppler boosts, strong self-lensing,
frame-dragging, and the differential gravitational redshift arising from the
stellar oblateness. In this Letter, we calculate line profiles taking into
account the first two effects, which is accurate for rotation rates less than
the breakup frequency. We show that the line profiles are not only broadened
and weakened but are also significantly asymmetric, and allow for an
independent measurement of both the mass and the radius of the neutron star.
Furthermore, we investigate the case when a fraction of the neutron star
surface contributes to the emission and find that the line profiles are
typically doubly peaked. We discuss the implications of our results for
searches for line features in the spectra of isolated neutron stars and X-ray
bursters. We finally assess the systematic uncertainties introduced by the line
asymmetry in inferring the compactness of neutron stars from the detection of
redshifted lines.
