Combined transmission spectrum of KELT-11 b in the sodium doublet wavelength range, using Epoch 2 and Epoch 3 merged from HARPS observations. The vertical blue dashed lines show the planetary rest frame transition wavelength of the Na I doublet, the horizontal one is the white-light radius. Top: transmission spectrum and Gaussian fit of the lines. The gray line represents the unbinned resulting transmission spectrum. The black points show the binned spectrum with a 0.1 Å step. The red line is the Gaussian fit to the unbinned transmission spectrum. Bottom: residuals from the Gaussian fit to the transmission spectrum. The gray line represents the residuals at each wavelength and is binned in the same way as the transmission spectrum.