Only eleven O-type stars have been confirmed to possess large-scale organized magnetic fields .
The presence of a -600 G longitudinal magnetic field in the O9.7 V star HD 54879 with a lower limit of the dipole strength of \sim 2 kG , was discovered a few years ago in the framework of the ESO large program ‘ ‘ B-fields in OB stars ’ ’ .
Our FORS 2 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 -300 G down to about -90 G. 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 -833 G. The inspection of the FORS 2 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 10 km s ^ { -1 } .
Different scenarios are discussed in an attempt to interpret our observations .
The FORS 2 radial velocity measurements indicate that HD 54879 is a member of a long-period binary .