Theoretical absorption spectra of HD 209458 b computed with the out-of-transit spectrum as a function of the wavelength in the planetary rest frame with and without sodium atmosphere. The blue points are ESPRESSO data from Casasayas-Barris et al. (2021). Upper panel: mean of the theoretical absorption spectra between contact times T₂ and T₃. Lower panel: theoretical absorption spectrum at contact times T₂ and T₃.