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We present the results of high-resolution spectropolarimetric observations ofthe optically dominant component in the rare hydrogen-deficient binary systemupsilon Sgr. Only a small number of such systems in a very late phase of heliumshell burning are currently known. The mass transfer from the donor star inbinary systems usually leads to the stripping of its hydrogen envelope.Consequently, since the mass of the secondary increases, it appearsrejuvenated. Using a few ESO FORS1 low-resolution spectropolarimetricobservations of this system, Hubrig et al. announced in 2009 the presence of amagnetic field of the order of -70 - -80G. Here we report on more recenthigh-resolution ESO HARPS spectropolarimetric observations showing that theprimary in upsilon Sgr is a spectrum variable star and possesses a weakmagnetic field of the order of a few tens of Gauss. The detection of a magneticfield in this rare hydrogen-deficient binary is of particular interest, as suchsystems are frequently discussed as probable progenitors of core-collapsesupernovae and gravitational-wave sources. Future magnetic studies of suchsystems will be worthwhile to gain deeper insights into the role of magneticfields in the evolution of massive stars in binary systems.

The magnetic field of the stripped primary in the upsilon Sgr system, a member of the rare class of hydrogen-deficient binaries