Stellar wind variability in OB stars and its effect on mass-loss determinations

Virtually all stellar winds of OB stars exhibit variability in one spectral diagnostic or another. This review focuses primarily on UV spectroscopic signatures of this variability and on studies that have exploited it to reveal the presence of large-scale velocity and density structures in the winds. The review includes demonstrations that the physical structures responsible for the variations appear to be large, spatially coherent and rooted in the stellar photospheres. Furthermore, evidence is summarised that these structures can be optically thick to resonance line radiation and are dense enough to modulate the X-ray fluxes that originate in the wind. Also discussed, is how the structures can affect wind line formation and, as a result, the precision by which mass-loss rates can be determined. The potential of millimetre and radio continuum observations to constrain the radial stratification of wind clumping is highlighted, as a future path to reliable empirical determinations of massive star mass-loss rates.