  A simple model to explain the phase-dependence of the cyclotron absorption
line observed in many X-Ray pulsars is presented. It includes several
relativistic effects, namely gravitational redshift, gravitational light
deflection, and - most important - doppler shift of the photon energy in the
infalling plasma. It is shown that previous estimates of neutron star magnetic
fields neglecting the last effect give results which are too small by about a
factor of two. Finally the developed equations are used to determine the
neutron star radius and magnetic field of Her X-1, but the large uncertainties
in the geometry prevent the results from seriously constraining the parameters.
