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From the radial velocities of the N  iv λ4058 and He  ii λ4686 emission lines, and the N  v λ4604‐20 absorption lines, determined in digital spectra, we report the discovery that the X‐ray bright emission line star Wack 2134 (= WR 21a) is a spectroscopic binary system with an orbital period of  31.673 ± 0.002 d . With this period, the N  iv  and He  ii  emission and N  v  absorption lines, which originate in the atmosphere of the primary component, define a rather eccentric binary orbit  ( e = 0.64 ± 0.03) . The radial velocity variations of the N  v  absorptions have a lower amplitude than those of the He  ii  emission. Such a behaviour of the emission line radial velocities could be due to distortions produced by a superimposed absorption component from the companion. High‐resolution echelle spectra observed during the quadrature phases of the binary show H and He  ii  absorptions of both components with a radial velocity difference of about 541 km s −1 . From this difference, we infer quite high values of the minimum masses, of about 87 and  53 M ⊙   for the primary and secondary components, respectively, if the radial velocity variations of the He  ii  emission represent the true orbit of the primary. No He  i  absorption lines are observed in our spectra. Thus, the secondary component in the Wack 2134 binary system appears to be an early O‐type star. From the presence of H, He  ii  and N  v  absorptions, and N  iv  and C  iv  emissions, in the spectrum of the primary component, it most clearly resembles those of Of/WNLha‐type stars.

The very massive X‐ray bright binary system Wack 2134 (= WR 21a) ★