Na I reference spectrum of HD 172555 (red line) in the Na I D2 and D1 lines (left and right panels, respectively) using the same caption as that of Fig. 1. Velocities are expressed in the heliocentric rest frame. No significant Na I absorption feature appears at the star’s radial velocity (v ~ 2 km s^-1), suggesting the CS origin of the absorption detected in the Ca II lines at this velocity. Conversely, the strongest absorption in Na I is seen at v ~  − 19 km s^-1 and is most likely of interstellar origin. The narrow absorption lines seen in most of the spectra (black dots) and not in the calculated reference spectrum are due to atmospheric water.