We present 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 % to the total flux at \lambda 6300 and orbits the neutron star at K _ { 2 } = 287 \pm 12 km s ^ { -1 } .
Contrary to other soft X-ray transients ( SXTs ) , the H _ { \alpha } emission is almost exactly in antiphase with the velocity curve of the optical companion .
Using the light-center technique we obtain K _ { 1 } = 140 \pm 27 km s ^ { -1 } and hence q = K _ { 1 } / K _ { 2 } = M _ { 2 } / M _ { 1 } = 0.49 \pm 0.10 .
This , combined with a previous determination of the inclination angle ( i = 73 ^ { \circ } \pm 4 ) yields M _ { 1 } = 1.55 \pm 0.31 M _ { \odot } and M _ { 2 } = 0.76 \pm 0.22 M _ { \odot } . M _ { 2 } agrees well with the observed spectral type .
Doppler tomography of the H _ { \alpha } emission shows a non-symmetric accretion disc distribution mimicking that seen in SW Sex stars .
Although we find a large systemic velocity of -110 \pm 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 .