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The O-type object 9 Sgr is a well-known synchrotron radio emitter. Thisfeature is usually attributed to colliding-wind binary systems, but 9 Sgr waslong considered a single star. We have conducted a long-term spectroscopicmonitoring of this star to investigate its multiplicity and search for evidencefor wind-wind interactions. Radial velocities are determined and analysed usingvarious period search methods. Spectral disentangling is applied to separatethe spectra of the components of the binary system. We derive the first everorbital 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 highlyeccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emissionlines that could be formed in the wind interaction zone in agreement with theexpected properties of the interaction in such a wide system. Our resultsprovide further support to the paradigm of synchrotron radio emission fromearly-type stars being a manifestation of interacting winds in a binary system.

9 Sgr: uncovering an O-type spectroscopic binary with an 8.6 year period