  Context: Stellar evolution models are highly dependent on accurate mass
estimates, especially for high-mass stars in the early stages of evolution. The
most direct method for obtaining model-independent masses is derivation from
the orbit of close binaries. Aims: To derive the first astrometric+RV orbit
solution for the single-lined spectroscopic binary MWC 166 A, based on CHARA
and VLTI near-infrared interferometry over multiple epochs and ~100 archival
radial velocity measurements, and to derive fundamental stellar parameters from
this orbit. We also sought to model circumstellar activity in the system from
K-band spectral lines. Methods: We geometrically modelled the dust continuum to
derive astrometry at 13 epochs and constrain individual stellar parameters. We
used the continuum models as a base to examine differential phases,
visibilities and closure phases over the Br-$\gamma$ and He-I emission lines.
Results: Our orbit solution suggests a period of $367.7\pm0.1$ d, twice as long
as found with previous RV orbit fits, subsequently constraining the component
masses to $M_1=12.2\pm2.2 M_\odot$ and $M_2=4.9\pm0.5 M_\odot$. The
line-emitting gas was found to be localised around the primary and is spatially
resolved on scales of ~11 stellar radii, with the spatial displacement between
the line wings consistent with a rotating disc. Conclusions: The large radius
and stable orientation of the line emission are inconsistent with
magnetospheric or boundary-layer accretion, but indicate an ionised inner gas
disk around MWC 166 Aa. We observe line variability that could be explained
either with generic line variability in a Herbig star disc or V/R variations in
a decretion disc. We also constrained the age of the system to
~$(7\pm2)\times10^5$ yr, consistent with the system being comprised of a
main-sequence primary and a secondary still contracting towards the main
sequence.
