  NU Ori is a massive spectroscopic and visual binary in the Orion Nebula
Cluster, with 4 components: Aa, Ab, B, and C. The B0.5 primary (Aa) is one of
the most massive B-type stars reported to host a magnetic field. We report the
detection of a spectroscopic contribution from the C component in
high-resolution ESPaDOnS spectra, which is also detected in a Very Large
Telescope Interferometer (VLTI) dataset. Radial velocity (RV) measurements of
the inner binary (designated Aab) yield an orbital period of 14.3027(7) d. The
orbit of the third component (designated C) was constrained using both RVs and
interferometry. We find C to be on a mildly eccentric 476(1) d orbit. Thanks to
spectral disentangling of mean line profiles obtained via least-squares
deconvolution we show that the Zeeman Stokes $V$ signature is clearly
associated with C, rather than Aa as previously assumed. The physical
parameters of the stars were constrained using both orbital and evolutionary
models, yielding $M_{\rm Aa} = 14.9 \pm 0.5 M_\odot$, $M_{\rm Ab} = 3.9 \pm 0.7
M_\odot$, and $M_{\rm C} = 7.8 \pm 0.7 M_\odot$. The rotational period obtained
from longitudinal magnetic field $\langle B_z \rangle$ measurements is $P_{\rm
rot} = 1.09468(7)$ d, consistent with previous results. Modeling of $\langle
B_z \rangle$ indicates a surface dipole magnetic field strength of $\sim 8$ kG.
NU Ori C has a magnetic field strength, rotational velocity, and luminosity
similar to many other stars exhibiting magnetospheric H$\alpha$ emission, and
we find marginal evidence of emission at the expected level ($\sim$1% of the
continuum).
