Left panel: Q/I profile of the radiation emitted across a ²S − ²P transition (the level energies and transition probabilities are those of the Mg iih and k lines) as obtained for different values of the parameter ϵ′ (indicated in the plot). The arrows point to the wavelength positions of the two lines. Right panel: zoom of the line-core region of the 1/2−3/2 transition. We consider the radiation emitted at μ = 0.1 by a slab located 0.03 stellar radii above the surface, and with an optical depth (at the line-center frequency of the 1/2−1/2 transition) Δτ = 0.5. We solve the full non-LTE radiative transfer problem within the slab, the boundary condition being the stellar radiation illuminating the slab from below (limb-darkening effects are neglected). We consider a Doppler width of 26 mÅ, corresponding to a temperature of 10⁴ K and a microturbulent velocity of 1 km s^-1. We include the effect of an unpolarized continuum characterized by an opacity 10⁸ times less than the line opacity at the line-center frequency of the 1/2−1/2 transition. The reference direction for positive Q is the parallel to the closest limb.