2007MNRAS.380.1219C

Phase-resolved medium-resolution VLT spectroscopy of the low-mass X-ray binary GX 9+9 has revealed narrow Ciii emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230 ±35 km/s. We identify the origin of these lines as coming from the surface of the donor star, thereby providing the first estimate of the mass function of f(M₁) ≥ 0.22M_☉. Rotational broadening estimates together with assumptions for the mass donor give 0.07 ≤ q ≤ 0.35 and 182 ≤ K₂≤ 406 km/s. Despite a low-mass ratio, there is no evidence for a superhump in our data set. Doppler maps of GX 9+9 show the presence of a stream overflow, either in the form of material flowing downward along the accretion disc rim or in a similar fashion as occurs in high mass transfer rate cataclysmic variables known as the SWSex stars. Finally, we note that the Bowen region in GX 9+9 is dominated by Ciii instead of Niii emission as has been the case for most other X-ray binaries.