Effect of the C iii UV lines on the formation of C iii 5696. Left: initial C iii 5696 spectrum (solid) and spectra from models in which several C iii UV lines have been artificially removed. Right: departure coefficient of the C iii 5696 lower and upper levels in the initial model (solid lines), the model without C iii 884 Å (dashed lines) and the model without C iii 574 Å (dotted lines). Departure coefficients are plotted as a function of Rosseland opacity. The dot-dashed line at the bottom indicates the C iii 5696 formation region expressed in terms of the contribution function as a function of τ_Rosseland.