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The first FORS2 spectropolarimetric observation of the longitudinal magneticfield of HD54879 of the order of -600G with a lower limit of the dipolestrength of ~2kG dates back to 2014. Since then observations showed a gradualdecrease of the absolute value of the mean longitudinal magnetic field. In thecourse of the most recent monitoring of HD54879 using FORS2 spectropolarimetricobservations from 2017 October to 2018 February, a longitudinal magnetic fieldstrength change from about -300G down to about -90G was reported. A suddenincrease of the absolute value of the mean longitudinal magnetic field and anaccompanying spectral variability was detected on 2018 February 17. New FORS2spectropolarimetric data obtained from 2018 December to 2019 February confirmthe very slow magnetic field variability, with the field decreasing from about150G to -100G over two months. Such a slow magnetic field variability, relatedto the extremely slow rotation of HD54879, is also confirmed usinghigh-resolution HARPSpol and ESPaDOnS spectropolarimetry. The re-analysis ofthe FORS2 polarimetric spectra from 2018 February indicates that the previouslyreported field increase and the change of the spectral appearance was caused byimproper spectra extraction and wavelength calibration using observationsobtained at an insufficient signal-to-noise ratio. The presented properties ofHD54879 are discussed in the context of the Of?p spectral classification.

The very slow rotation of the magnetic O9.7V star HD54879