The effect of non-coherent scattering by electrons and atoms on line formation in early-type stars has been studied using model atmospheres. Variability of the physical parameters has been fully in- cluded. The effect of the non-coherency of the scattering by electrons is noticeable for a line like He ii X4200 but not for a much stronger one like He ii X4686. The solution in the core is dominated by the degree to which scattering (by atoms) occurs, while the wings are unaffected. The observed profile of He ii X4686 in 10 Lac can be fitted if the core of the line is deepened by scattering and broadened by rotation
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Auer et al. (1968) studied this question.