  We report the detection of a strong, organized magnetic field in the
secondary component of the massive O8III/I+O7.5V/III double-lined spectroscopic
binary system HD 47129 (Plaskett's star), in the context of the Magnetism in
Massive Stars (MiMeS) survey. Eight independent Stokes $V$ observations were
acquired using the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii
Telescope and the Narval spectropolarimeter at the T\'elescope Bernard Lyot.
Using Least-Squares Deconvolution we obtain definite detections of signal in
Stokes $V$ in 3 observations. No significant signal is detected in the
diagnostic null ($N$) spectra. The Zeeman signatures are broad and track the
radial velocity of the secondary component; we therefore conclude that the
rapidly-rotating secondary component is the magnetized star. Correcting the
polarized spectra for the line and continuum of the (sharp-lined) primary, we
measured the longitudinal magnetic field from each observation. The
longitudinal field of the secondary is variable and exhibits extreme values of
$-810\pm 150$ G and $+680\pm 190$ G, implying a minimum surface dipole polar
strength of $2850\pm 500$ G. In contrast, we derive an upper limit ($3\sigma$)
to the primary's surface magnetic field of 230 G. The combination of a strong
magnetic field and rapid rotation leads us to conclude that the secondary hosts
a centrifugal magnetosphere fed through a magnetically confined wind. We
revisit the properties of the optical line profiles and X-ray emission -
previously interpreted as a consequence of colliding stellar winds - in this
context. We conclude that HD 47129 represents a heretofore unique stellar
system - a close, massive binary with a rapidly rotating, magnetized component
- that will be a rich target for further study.
