Line source functions for the N iii/O iii resonance lines at 374 Å, for two versions of model s6a, as calculated by cmfgen. Upper panel: N iii “decoupled” (dashed/blue); O iii “decoupled” (dashed-dotted/red); N iii and O iii “coupled” (solid, identical). Lower panel: source function ratios. Solid: N iii (“coupled”)/N iii (“decoupled”); dashed: O iii (“coupled”)/O iii (“decoupled”). The optical triplet lines are formed in the range indicated by the dotted lines.