Transmission spectrum of KELT-11 b in the Na doublet wavelength range for each transit observed with HARPS, at the same scale. The vertical blue dashed lines show the planetary rest frame transition wavelength of the Na doublet, the horizontal one is the white-light radius $R_{\text{p}}^2\text{/}{R}_{\text{*}}^2$. The gray line represents the unbinned resulting transmission spectrum. The black points show the binned spectrum with a 0.1 Å step.