Panel a) The Rosseland-mean opacities, κ_R    [cm²   g^-1] , as a function of lgT for Castelli & Kurucz (2003) (marked as CK line) and Lanz & Hubeny (2007) (LH line) models of  K and lgg = 4.0 in comparison with OPAL (OPAL line) and OP (OP line) stellar envelope models. A non-LTE model calculated for a more complex model of O V ion (cf. Sect. 4.3) is also shown (T line). Panel b) The same as panel a), but on κ_R vs. lgR diagram. Panel c) The same as panel a), but for the radiation pressure gradients (in cgs units) as a function of lgT.