tokens: pronounced, indicating an increase by ∼100 km s −1 . Remarkably, we observed at the epoch of the sudden magnetic field increase (indicated by the red triangle) that the radial velocity of all spectral lines dropped by several 10 km s −1 . To better highlight the observed changes in radial velocities, we show in Fig. 9 the radial velocity shift correlations between the lines belonging to different elements. Figure 7. Radial velocities measured for different spectral lines in the observations obtained between 2014 and 2018. Figure 8. Radial velocities measured for different spectral lines in the observations obtained between 2017 and 2018. Figure 9. Radial velocity correlations for hydrogen and He i lines. The observed dispersion of the radial velocity measurements in these figures is probably related to β Cephei-like pulsations. The atmospheric parameters presented by Castro et al. ( 2015 ) suggest that HD 54879 has already slightly evolved from the ZAMS and is passing through the β Cephei instability strip. As mentioned above, Järvinen et al. ( 2017 ) were not able to detect significant velocity shifts in high-resolution HARPSpol observations of HD 54879, most likely due to much longer exposure times, of the order of 1–3 h. During the long HARPSpol exposures, any spectral variability is smeared over the pulsation cycle and difficult to detect. In contrast, FORS 2 observations carried out using an 8 m telescope have a duration of only 10–20 min and are expected to be more strongly affected by the β Cephei-like pulsations. According to Telting et al. ( 2006 ), at least half of the late-O to early-B solar-neighbourhood stars that are located in the β Cephei instability strip are actually pulsating in radial and/or non-radial modes. With respect to the remarkable change of the spectral appearance of HD 54879 during the sudden magnetic field increase, it is important to mention that a recent study of the most strongly magnetized O-type star NGC 1624-2 using UV observations with HST /COS revealed similar dramatic variations in the resonance line profiles between rotation phases when looking nearly at the magnetic pole and those when looking nearly at the magnetic equator (see fig. 1 in David-Uraz et al. 2018 ). Similar to our observations of HD 54879, these line profiles show at these two rotation phases very different characteristics including the line intensity, line shape and width, and radial velocity. It should however be noted that the variability of the UV resonance lines in massive stars is usually related to the stellar wind or to the magnetosphere, while for HD 54879 we observe line profile changes in the photospheric lines. It cannot be excluded that the observed changes in radial velocity are caused by an invisible wide companion. Indeed, Castro et al. ( 2015 ) compared their radial velocity measurement from one HARPSpol spectrum with other measurements in the literature and concluded that HD 54879 could be a member of a long-period
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