Intensity ratios of recombination lines. From top to bottom: I(C II λ5342)/I(C II λ4267), I(C II λ6151)/I(C II λ4267), I(C II λ6462/ I(C II λ4267), and I(C II λ9903)/I(C II λ4267) ratios of all analyzed regions: galactic PNe (red circles), galactic H II regions (black diamonds), extragalactic H II regions (green squares), and photoionized Herbig-Haro objects (purple stars) as a function of the degree of ion-ization P. The red band shows the theoretical ratio expected for nebular conditions consisting of a temperature in the range 4000–15 000 K and a density 10³ cm⁻³ assuming the atomic data of Pequignot et al. (1991) (P91, only available for C II 4267 and 9903) while the blue band shows the theoretical ratio expected when adopting the atomic data of Davey et al. (2000) (D00).