Effect of the metallicity Z on the formation of C iii 5696 at T_eff = 30 500 K. Left: spectra of C iii 5696 at Z_⊙ (solid), 1/5th Z_⊙ (dashed) and 1/5th Z_⊙ but with C/H at the solar value (dotted). Right: departure coefficient of the C iii 5696 lower and upper levels at Z_⊙ (solid lines), 1/5th Z_⊙ (dashed lines) and 1/5th Z_⊙ with C/H at the solar value (dotted). The dot-dashed line indicates the line formation region.