We study the K-correction for the case of emission lines formed in the X-ray-illuminated atmosphere of a Roche lobe-filling star. We compute the K-correction as a function of the mass ratio q and the disk flaring angle α using a compact binary code in which the companion's Roche lobe is divided into 105 resolution elements. We also study the effect of the inclination angle in the results. We apply our model to the case of the neutron star low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity Kem=300+/-8 km s-1 has been measured by Casares et al. Our numerical results, combined with a previous determination of system parameters, yields 1.61 Msolar<=MNS<=2.32 Msolar and 0.44 Msolar<=M2<=0.56 Msolar for the two binary components (i.e., 0.24<=q<=0.27), which provide compelling evidence for a massive neutron star in this system. We also discuss the implications of these masses into the evolutionary history of the binary.