Quiescent C II lines of GJ 3470. Observed line profiles were corrected for their offsets with respect to their expected rest wavelength and averaged over Visits D, E, and F (black histogram). The green spectrum is the best fit for the intrinsic stellar lines at Earth’s distance. It yields the blue profile after absorption by the ISM and the red profile after convolution with the COS LSF. The ISM absorption profile is plotted as a dotted black line (scaled to the panel vertical range).