Rosseland-mean opacity κ_R [ cm² g^-1 ] vs. lgT taken from the Castelli & Kurucz (2003) atmosphere models of lgg = 5.0, T_eff = 15 000, 20 000, 25 000, 30 000 K (left panel) and T_eff = 35 000,40 000,45 000 K (right panel) in comparison with envelope models calculated with the OPAL data. The atmosphere models corresponding to the microturbulent velocity ξ = 0 (taken from the Castelli website) were plotted as lines with pluses. Both the CK and OPAL models were calculated for X = 0.7347 and Z = 0.0169.