Discovery of a magnetic field in the B pulsating system HD 1976

Aims: A magnetic field can have a strong impact on the evolution of a binary star. However, only a dozen magnetic OB binaries are known as of today and are available to study this effect, including some very few magnetic pulsating spectroscopic OB binaries. We checked for the presence of a magnetic field in the B5IV hierarchical triple system HD 1976 with spectropolarimetric data obtained with Narval at the Bernard Lyot Telescope (TBL). Methods: We used orbital parameters of HD 1976 available in the literature to disentangle the Narval intensity spectra. We computed Stokes V profiles with the least squares deconvolution technique to search for magnetic signatures. We then derived an estimate of the longitudinal magnetic field strength for each observation and for various line lists. Results: Our disentangling of the intensity spectra shows that HD 1976 is a double-lined spectroscopic (SB2) binary, the lines of the secondary component are about twice as broad as those of the primary component. We did not identify the third component. Moreover, we find clear magnetic signatures in the spectropolarimetric measurements of HD 1976 that seem to be associated with the primary component. We conclude that HD 1976 is a magnetic slowly pulsating double-lined spectroscopic binary star with an undetected third component. It is the second such example known (the other is HD 25558).Accepted in A&A Letter to the Editor, 4 pages, 2 figures, 2 tablesstatus: publishe