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We present strong evidence for a rotation period of 2.0406 d of the O giant$\xi$ Persei, derived from the NIV $\lambda$1718 wind line in 12 yr of IUEdata. We predict that $\xi$ Per has a magnetic dipole field, with superposedvariable magnetic prominences. Favorable dates for future magnetic measurementscan be predicted. We also analysed time-resolved He II 4686 spectra from acampaign in 1989 by using the same simplified model as before for $\lambda$Cephei, in terms of multiple spherical blobs attached to the surface, calledstellar prominences (Sudnik & Henrichs 2016). These represent transientmultiple magnetic loops on the surface, for which we find lifetimes of mostlyless than 5 h.

Multiple, short-lived "stellar prominences" on the O giant $\xi$ Persei: a magnetic star?