The UV line spectrum of the soft X-ray transient XTE J1118+480: a CNO processed core exposed

We compare UV spectra of the recent soft X-ray transients XTE J1118+480 and XTE J1859+226. The emission line strengths in XTE J1118+480 strongly suggest that the accreting material has been CNO processed. We show that this system must have come into contact with a secondary star of about 1.5 solar masses, and an orbital period ~15hr, very close to the bifurcation value at which nuclear and angular momentum loss timescales are similar. Subsequent evolution to the current period of 4.1 hr was driven by angular momentum loss. In passing through a period of 7.75 hr the secondary star would have shown essentially normal surface abundances. XTE J1118+480 could thus represent a slightly later evolutionary stage of A0620-00. We briefly discuss the broad Ly alpha absorption wings in XTE J1118+480.