On the formation of the N III λλ4634−4640−4642 emission lines in hot massive stars (here: model s6a, see Table 2). Left: optical emission lines (no broadening, no degrading) for various configurations of the EUV N III resonance lines (around 374 Å) coupled to the upper level (3d) of the optical transitions. See legend, and text. “[X]” means ɛ_X. Right: corresponding EUV radiation field, J_ν(τ_Ross = 0.24). The insert displays the decisive region, and participating lines. Red: N III resonance lines (three components); green: overlapping O III resonance line (out of a multiplet of 6 components); blue: Fe V line at 374.245 Å. We note that 0.01 Å correspond to 8 km s ⁻¹.