  Phase-resolved medium resolution VLT spectroscopy of the low mass X-ray
binary GX9+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(M_1)>=0.22M_sun. Rotational broadening estimates together with assumptions
for the mass donor give 0.07<q<0.35 and 182<K_2<406 km/s. Despite a low mass
ratio, there is no evidence for a superhump in our dataset. Doppler maps of
GX9+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 GX9+9 is dominated by CIII instead of
NIII emission as has been the case for most other X-ray binaries.
