  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 IUE
data. We predict that $\xi$ Per has a magnetic dipole field, with superposed
variable magnetic prominences. Favorable dates for future magnetic measurements
can be predicted. We also analysed time-resolved He II 4686 spectra from a
campaign in 1989 by using the same simplified model as before for $\lambda$
Cephei, in terms of multiple spherical blobs attached to the surface, called
stellar prominences (Sudnik & Henrichs 2016). These represent transient
multiple magnetic loops on the surface, for which we find lifetimes of mostly
less than 5 h.
