¹²CO⁺ and ¹³CO⁺ theoretical spectra superimposed on the observational data obtained on comet C/2016 R2. The relative intensity of ¹³CO⁺ emission lines are based on the ¹²CO⁺ fit of the emission lines (i.e., a theoretical ratio ¹²CO⁺/¹³CO⁺ = 1). They do not respect the intensities observed in this comet, the purpose of this plot being only to show the wavelength shift between the emission lines of these two isotopologues.