  High-resolution circular spectropolarimetric observations, obtained with
ESPaDOnS in the context of the BinaMIcS Large Program, have revealed a magnetic
field in the B3V secondary component of the SB2 binary system $\epsilon$ Lupi
(B2/B3). As the B2V primary is already known to be magnetic, this is the first
detection of a magnetic field in both components of an early-type binary
system. The longitudinal magnetic field of the primary is $\sim -200$ G; that
of the secondary $\sim +100$ G. Observations can be approximately reproduced by
a model assuming the magnetic axes of the two stars are anti-aligned, and
roughly parallel to their respective rotation axes. Estimated magnetospheric
radii indicate a high probability that their magnetospheres are interacting. As
many of the arguments for the different proposed formation scenarios of fossil
magnetic fields rely upon evidence drawn from investigations of close binaries,
in particular the rarity of magnetic ABO stars in close binaries and the
previous absence of any known close binary with two magnetic, massive stars,
this discovery may be an important new constraint on the origin of fossil
magnetic fields.
