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Most O and many B stars show unexplained cyclical variability in theirspectral lines, i.e. modulation on the rotational timescale, but not strictlyperiodic. The variability occurs in the so-called discrete absorptioncomponents (DACs) that accelerate through the UV-wind line profiles and also inmany optical lines. For such OB stars no dipolar magnetic fields have beendetected with upper limits of ~300G. We investigate whether multiple magneticloops on the surface rather than non-radial pulsations or a dipolar magneticfield can explain the observed cyclical UV and optical spectral linevariability. We present time-resolved, high-resolution optical spectroscopy ofthe O6.5I(n)fp star lambda Cep. We apply a simplified phenomenological model inwhich multiple spherical blobs attached to the surface represent magnetic-loopstructures, which we call stellar prominences, by analogy with solarprominences. We compare the calculated line profiles as a function ofrotational phase, adopting a rotation period of 4.1d, with observed relativechanges in subsequent quotient spectra. We identify many periodicities inspectral lines, almost none of which is stable over timescales from months toyears. We show that the relative changes in various optical absorption andemission lines are often very similar. Our proposed model applied to the HeII4686 line can typically be fitted with 2-5 equatorial blobs with lifetimesbetween ~1 and 24h. Given the irregular timescales involved, we propose thatthe azimuthal distribution of DACs correspond to the locations of stellarprominences attached to the surface. This could explain the observedvariability of optical and UV lines, and put constraints on the strength andlifetime of these structures, which can be compared with recent theoreticalpredictions, in which bright magnetic surface spots are formed by the action ofthe subsurface convection zone.

Multiple short-lived stellar prominences on O stars: The O6.5I(n)fp star lambda Cephei