HD5550 is a spectroscopic binary composed of two A stars observed with Narval at TBL in the frame of the BinaMIcS (Binarity and Magnetic Interactions in various classes of Stars) Large Program. One component of the system is found to be an Ap star with a surprisingly weak dipolar field of ~65 G. The companion is an Am star for which no magnetic field is detected, with a detection threshold on the dipolar field of ~40 G. The system is tidally locked, the primary component is synchronised with the orbit, but the system is probably not completely circularised yet. This work is only the second detailed study of magnetic fields in a hot short-period spectroscopic binary. More systems are currently being observed with both Narval at TBL and ESPaDOnS at CFHT within the BinaMIcS project, with the goal of understanding how magnetism can impact binary evolution and vice versa.
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