line can be either decoded using a normal (’smooth’) line opacity and a small mass-loss rate or a small (’porosity’) line opacity and a higher mass-loss rate. In this paper we argue that the optical depths of the Narrow Absorption Components (NACs) commonly seen in OB stars provide the most reliable (i.e. porosity-free) measure of mass-loss rate in UV resonance lines.

Narrow absorption components (NACs)
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Time-series UV studies of Galactic OB stars have repeatedly demonstrated that wind line profile variability may often be interpreted in terms of blueward-migrating discrete absorption components (DACs), which recur on time scales of days, and which narrow (in velocity space) as they accelerate towards the shortward profile edge (see e.g. Kaper et al. 1996; Prinja et al. 2002). Since the timescale of this UV line profile variability is (in a few case studies) cyclical and commensurate with the (maximum) rotation period of the star, this suggests an origin that is somehow connected to the stellar surface. A promising explanation for the observed cyclical variability is provided by the so-called co-rotating interaction regions (CIRs): spiral structures formed by the interaction of gas streams flowing from the stellar surface with different velocities (Mullan, 1986). The formation 