Context : 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 .

Aims : We have conducted a long-term spectroscopic monitoring of this star to investigate its multiplicity and search for evidence for wind-wind interactions .

Methods : 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 .

Results : 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 .

Conclusions : 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 .