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Phase-resolved medium-resolution VLT spectroscopy of the low-mass X-ray binary GX 9+9 has revealed narrow C iii emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230 ± 35 km s-1. 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) ≥ 0.22 M⊙. Rotational broadening estimates together with assumptions for the mass donor give 0.07 ≤ q ≤ 0.35 and 182 ≤ K2 ≤ 406 km s -1. 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 SW Sex stars. Finally, we note that the Bowen region in GX 9+9 is dominated by C iii instead of N iii emission as has been the case for most other X-ray binaries. © 2007 RAS.

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Monthly Notices of the Royal Astronomical Society

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