  The optical spectrum of WR 138 exhibits emission lines typical of a WN6o star
and absorption lines from a rapidly-rotating OB star. Using a large set of
spectroscopic data, we establish a new orbital solution of the WN6o star based
on the radial velocities of highly-ionized nitrogen lines. We show that the
WN6o star moves on a 4.3 yr orbit with a comparatively low eccentricity of
0.16. The radial velocities of the OB star display considerable scatter. Our
best estimates of the velocities of He I absorption lines result in a
mass-ratio of $m_{\rm WN6o}/m_{\rm OB} = 0.53 \pm 0.09$. We disentangle the
spectra of the two stars and derive a projected rotational velocity of
$v\,\sin{i} = 350 \pm 10$ km s$^{-1}$ for the OB star. Our best orbital
parameters, combined with the Gaia parallax of WR 138, are at odds with a
previous interferometric detection of the companion, suggesting that there is
either a bias in this detection or that WR 138 is actually a triple system.
