A short and sudden increase of the magnetic field strength and the accompanying spectral variability in the O9.7V star HD54879

Only eleven O-type stars have been confirmed to possess large-scale organized magnetic fields. The presence of a -600G longitudinal magnetic field in the O9.7V star HD54879 with a lower limit of the dipole strength of ~2kG, was discovered a few years ago in the framework of the ESO large program "B-fields in OB stars". Our FORS2 spectropolarimetric observations from 2017 October 4 to 2018 February 21 reveal the presence of short and long-term spectral variability and a gradual magnetic field decrease from about -300G down to about -90G. Surprisingly, we discover on the night of 2018 February 17 a sudden, short-term increase of the magnetic field strength and measure a longitudinal magnetic field of -833G. The inspection of the FORS2 spectrum acquired during the observed magnetic field increase indicates a very strong change in spectral appearance with a significantly lower photospheric temperature and a decrease of the radial velocity by several 10km/s. Different scenarios are discussed in an attempt to interpret our observations. The FORS2 radial velocity measurements indicate that HD54879 is a member of a long-period binary.