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 \pm 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 ( M _ { 1 } ) \geq 0.22 M _ { \odot } .
Rotational broadening estimates together with assumptions for the mass donor give 0.07 \leq q \leq 0.35 and 182 \leq K _ { 2 } \leq 406 km s ^ { -1 } .
Despite a low mass ratio , there is no evidence for a superhump in our dataset .
Doppler maps of GX 9 + 9 show the presence of a stream overflow , either in the form of material flowing downward along the accretion disk 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 .