Rotation period of the O giant ξ Persei: Coexistence of cyclic and periodic variability. A weakly magnetic candidate star

OB stars exhibit line profile variability, often over timescales associated with rotation; notably, in their ultraviolet wind-sensitive lines where `discrete absorption components' tend to recur cyclically. We searched for periodicity in spectral regions of the O7.5III(n)((f)) star formed very close to the star and least likely to be affected by doublet overlap or irregular transient phenomena. ξ,Persei We identified the low-velocity wind region probed by the łambda1718,Å line as the most uncontaminated spectral region for which a large and homogeneous dataset is available: 307 International Ultraviolet Explorer (IUE) spectra over 12 years and 11 STIS N IV (on board the Hubble Space Telescope) spectra taken 21 years later. We also studied 322 time-resolved łambda4686,Å spectra. We searched for periodicities in high-precision space photometry (MOST, BRITE, and TESS), covering a time span of 13 years. We also reconsidered X-ray studies He II with Chandra . A CLEAN analysis and subsequent weighted least-squares fit of the flux in this region resulted in a unique period of 2.040514 ± 0.000020,d, attributed to rotation. The phase of maximum flux in the N IV , , Hα, and line profiles coincide with the results of X-ray variability studies. We also found this period with low amplitude in the photometric data, albeit with large intrinsic scatter, and with a deviating maximum phase. Si IV He II Given the coherent periodic behaviour of several observables probing the region of the wind nearest the stellar surface, we propose that ξ,Per has an as-yet undetected weak global surface magnetic field. We excluded a rotation period of ∼ 4,d based on stellar parameters. The sinusoidal behaviour suggested that only one magnetic pole is visible, implying an inclination of i,∼,51^̧irc, and therefore, β łesssim 90^ ̧irc - i = 39^ ̧irc for the magnetic obliquity. We present a conceptual framework within which weak wind confinement by a global magnetic field can create a small magnetospheric disc, while allowing for the formation of more classic `corotating interaction regions' at higher magnetic latitudes. The periodic variations are then understood to result from the changing projected area of the magnetospheric disc as a function of rotational phase, both along the line of sight and off the stellar limb.