wind over a Sobolev length may be more complex than the Sobolev approximation assumes. In that case, a more sophisticated radiative transfer code would indeed be needed to perform the calculation, including a detailed prescription of the density/velocity structure (which in unknown). Alternatively, the surfaces of constant velocity in the wind could be porous over a Sobolev length. This is indeed the interpretation adopted in the present paper. One argument that favours this conjecture is that the absorption components of wind line profiles show extremely strong temporal variability while their emission components remain relatively stable. This can be naturally explained in terms of covering factors. Further, more sophisticated theoretical investigations seem to verify this notion Sundqvist et al. (2010). Of course both of these alternatives may be present and the wind may be complicated by a variety of velocity plateaus and density variations. Nevertheless for a wind with complex density and velocity structure in the radial direction, one should expect similar structures in the lateral directions. And this lateral structure can give rise to variable covering factors less than one.

Thus we can interpret a change in the apparent doublet ratio as a function of velocity as a change in 