2002MNRAS.329...29C

We present Very Large Telescope (VLT) low-resolution spectroscopy of the neutron star X-ray transient XTE J2123-058 during its quiescent state. Our data reveal the presence of a K7V companion which contributes 77 per cent to the total flux at λ6300 and orbits the neutron star at K_2=287±12km.s^-1. Contrary to other soft X-ray transients (SXTs), the H_α emission is almost exactly in antiphase with the velocity curve of the optical companion. Using the light-centre technique we obtain {formmu2} K_1=140±27km.s^-1 and hence q=K_1/K_2=M_2/M_1=0.49±0.10. This, combined with a previous determination of the inclination angle {formmu4} (i=73^○±4^○) yields M_1=1.55±0.31 M_☉ and {formmu6} M_2=0.76±0.22 M_☉. M₂ agrees well with the observed spectral type. Doppler tomography of the Hα emission shows a non-symmetric accretion disc distribution mimicking that seen in SW Sex stars. Although we find a large systemic velocity of - 110±8 km.s^-1 this value is consistent with the galactic rotation velocity at the position of J2123-058, and hence a halo origin. The formation scenario of J2123-058 is still unresolved.