  Many O and B stars show unexplained cyclical variability in their winds, i.e.
modulation of absorption features on the rotational timescale, but not strictly
periodic over longer timescales. For these stars no dipolar magnetic fields
have been detected, with upper limits below 300 G. Similar cyclical variability
is also found in many optical lines, which are formed at the base of the wind.
We propose that these cyclical variations are caused by the presence of
multiple, transient, short-lived, corotating magnetic loops, which we call
"stellar prominences". We present a simplified model representing these
prominences to explain the cyclical optical wind-line variability in the O
supergiant lambda Cephei. Other supporting evidence for such prominences comes
from the recent discovery of photometric variability in a comparable O star,
which was explained by the presence of multiple transient bright spots,
presumably of magnetic origin as well.
