We have computed the K-correction for emission lines formed on the irradiated face of the companion stars in X-ray binaries. Our results, combined with the detection of high excitation emission lines arising from the donor star can provide the first mass determinations for either neutron stars or black holes in active Low Mass X-ray Binaries (LMXBs). Here we present preliminary results of a high spectral resolution VLT+UVES campaign on the LMXB X-ray pulsar X1822-371. We measure a emission line velocity of Kem = 280+/-3 km s-1. The application of the K-correction to this Kem together with previous determinations of system parameters yields 1.52Msolar