this interaction is with the diffuse, scattered component due to the blue component from the far side of the star, which is still weak at the red NAC velocity and will not introduce a substantial error to the optical depths.

As in our previous studies (e.g. Massa et al. 2003) we derived the radial optical depths from the observed Si[iv]{} line profiles by using the Sobolev with Exact Integration (SEI) method (Lamers et al. 1997), modified to allow the $\tau_{\rm{rad}}(w)$ values to be determined using a histogram of 21 variable bins in velocity space. We note that Bouret et al. (2012) have recently demonstrated that the SEI method is extremely accurate for wind profiles. The SEI approach offers two key advantages for the present study: (i) It is free of assumptions of how the wind material or its ionization state are distributed, thus making it an ideal tool for empirical investigations; (ii) It is extremely fast, so that it can be used in non-linear least squares routines, where profiles must be computed hundreds of times.

Our approach is to analyse as large a number of stars as possible and then examine the resulting parameters to search for systematics. Such a 