  The O-type object 9 Sgr is a well-known synchrotron radio emitter. This
feature is usually attributed to colliding-wind binary systems, but 9 Sgr was
long considered a single star. We have conducted a long-term spectroscopic
monitoring of this star to investigate its multiplicity and search for evidence
for wind-wind interactions. Radial velocities are determined and analysed using
various period search methods. Spectral disentangling is applied to separate
the spectra of the components of the binary system. We derive the first ever
orbital solution of 9 Sgr. The system is found to consist of an O3.5 V((f+))
primary and an O5- 5.5 V((f)) secondary moving around each other on a highly
eccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emission
lines that could be formed in the wind interaction zone in agreement with the
expected properties of the interaction in such a wide system. Our results
provide further support to the paradigm of synchrotron radio emission from
early-type stars being a manifestation of interacting winds in a binary system.
