mass-loss rates are calculated with a Monte Carlo (MC) code that is used to follow the fate of a large number of photons from below the stellar photosphere through the wind, and that calculates the radiative acceleration of the wind material. The core of the approach is that the cumulative loss of radiative energy equals the gain in kinetic energy of the wind material (Abbott & Lucy 1985). We assume that each individual photon scatters coherently in the co-moving frame. In the observer’s frame, this implies that each photon-ion interaction results in an exchange of momentum and energy (except for the case that the incident and emerging photon happen to have the same direction). As the absorptions and scatterings of photons in the wind depend on the wind density, this allows for a derivation of a globally consistent mass-loss rate – for a pre-described flow structure. Our approach is described in detail in Vink et al. (1999). We note that as the kinetic wind energy scales with ${\mbox{$v_{\infty}$}}^{2}$, the derived mass-loss rates depend on the choice of the terminal flow velocity.

The chemical species that are explicitly calculated in non-LTE are H, He, C, N, O and Si (see de 