Transmission spectrum of KELT-9 b at the location of eight strong lines of Fe II that were observed by Cauley et al. (2019). The grey points show the transmission spectrum at the native sampling of the HARPS-N instrument, while the black points are binned by 20 points. The transmission spectrum is obtained following the procedure of Wyttenbach et al. (2015), and combining all in-transit exposures of the two nights. The Doppler-shadow is removed by fitting the model described in Sect. 2 at the expected locations of the individual lines. The presence of these lines appears to be confirmed by our HARPS-N observations, but non-Gaussian systematic noise occurs at the ~ 10⁻³ level. This hinders the identification of all but the strongest Fe II lines.