HARPS transmission spectrum of WASP-127b for the sodium doublet in the planetary rest frame. Upper panel: transmission spectrum for all three (1, 2, 4) nights combined in full HARPS resolution is shown in grey, in black the same data is shown binned by ×20. The data has been corrected for tellurics, cosmics, telluric sodium emission contamination and low S/N remnants. The rest frame transition wavelengths are marked with blue dashed lines. We attempted a Gaussian fit to both potential sodium lines jointly, however, the fit was only successful for the D2 line. The fit is shown as a red line. We, therefore, fitted the continuum to the D1 line instead. Lower panel: residuals of the Gaussian fit.