  We report the detection of a strong, organised magnetic field in the O9IV
star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at
the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in
Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE
model atmospheres we determined that HD 57682 is a $17^{+19}_{-9}$ M$_{\odot}$
star with a radius of $7.0^{+2.4}_{-1.8}$ R$_\odot$, and a relatively low
mass-loss rate of $1.4^{+3.1}_{-0.95}\times10^{-9}$ M$_{\odot}$ yr$^{-1}$. The
photospheric absorption lines are narrow, and we use the Fourier transform
technique to infer $v\sin i=15\pm3$ km s$^{-1}$. This $v\sin i$ implies a
maximum rotational period of 31.5 d, a value qualitatively consistent with the
observed variability of the optical absorption and emission lines, as well as
the Stokes $V$ profiles and longitudinal field. Using a Bayesian analysis of
the velocity-resolved Stokes $V$ profiles to infer the magnetic field
characteristics, we tentatively derive a dipole field strength of
$1680^{+134}_{-356}$ G. The derived field strength and wind characteristics
imply a wind that is strongly confined by the magnetic field.

Solar and Stellar Astrophysics