WIND VARIABILITY IN O-TYPE STARS

An observational description is presented of the time-dependent behavi our of Discrete Absorption Components (DACs) in ultraviolet spectra of O-type stars. DACs represent the strongest manifestation of stellar-w ind variability, a characteristic property of radiation-driven winds. Wind variability sets in already close to the star, as is evidenced by the low-velocity variations in several subordinate ultraviolet and op tical lines. The observed characteristic timescale of the variability strongly suggests that stellar rotation plays a dominant role. We disc uss the DAC behaviour in terms of a model in which (localized) surface magnetic field regions impose an additional. boundary condition at th e base of the stellar wind, which may give rise to the-observed large- scale structures.