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The presence of a magnetic field in β Lyrae was firstly suggested by Babcock in 1958 and then confirmed by Skulsky in 1982. This kG-order large-scale organized magnetic field has been neglected in interpreting and modeling the large variety of phenomena presented by β Lyrae. Here, we present circular spectropolarimetry of β Lyrae showing that its magnetic field is variable with the orbital period and that it has changed in sign and strength between 1980 and 2000. Unfortunately, there are not enough data to conclude if a longer-time variability is super-imposed on the orbital period variability or if the field changes abruptly. This magnetic field, to our knowledge, is unique. Since we measured the magnetic field in metal lines of the brightest star of the system, we can conclude that this is the first magnetic B-type giant star. In this case, the magnetic field is significantly different from that of Magnetic Chemically Peculiar stars and the long-time-scale variability suggests the presence of a dynamo. However, we cannot rule out that the magnetic field measured on the brightest star is generated by the accretion disk, or that the magnetic field of the embedded star is so elongated in the orbital plane by the disk that it still has a significant strength even at the companion distance. In any case, the accretion disk is certainly related to the magnetic field of the β Lyrae system. We found that variations of the field in sign and strength corresponded to variations in the disk structure, as it is inferred from photometry and spectroscopy. Also, a magnetized disk explains the observed jet-like outflow from the β Lyrae system.

The magnetic field of the $\boldmath\beta$ Lyrae system: Orbital and longer time-scale variability