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Only eleven O-type stars have been confirmed to possess large-scale organizedmagnetic fields. The presence of a -600G longitudinal magnetic field in theO9.7V star HD54879 with a lower limit of the dipole strength of ~2kG, wasdiscovered a few years ago in the framework of the ESO large program "B-fieldsin OB stars". Our FORS2 spectropolarimetric observations from 2017 October 4 to2018 February 21 reveal the presence of short and long-term spectralvariability and a gradual magnetic field decrease from about -300G down toabout -90G. Surprisingly, we discover on the night of 2018 February 17 asudden, short-term increase of the magnetic field strength and measure alongitudinal magnetic field of -833G. The inspection of the FORS2 spectrumacquired during the observed magnetic field increase indicates a very strongchange in spectral appearance with a significantly lower photospherictemperature and a decrease of the radial velocity by several 10km/s. Differentscenarios are discussed in an attempt to interpret our observations. The FORS2radial velocity measurements indicate that HD54879 is a member of a long-periodbinary.

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