Left: temperature of optical depth for the different zones of a secondary irradiated by a cool white dwarf (see text for details of the model). The lines show the following situations (top to bottom): irradiation at the substellar point (green), at μ = 45° (dotted, red), μ = 60° (dashed, black) and the night-side, i.e. no irradiation (blue) (colour in electronic version only). Right: the structure of the temperature and radiation field in the irradiated atmospheres. The upper panel shows electron temperature T_elec, the middle panel the internal radiation flux u_H,   int, and the lower panel the external radiation flux u_H,   ext. See text for details of the definition of the fluxes. The different zone are marked as in the left panel (colour in electronic version only).