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

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.