HARPS transmission spectrum of WASP-76b centred around the sodium doublet in the planetary rest frame. Upper panel: light grey – full transmission spectrum for all nights combined in full resolution. Black – transmission spectrum binned by x20. The data has been corrected for tellurics and cosmics and shifted to the planetary rest frame. The absorption from the two Na I D lines from the planetary atmosphere can be seen clearly and the rest frame transition wavelengthsare marked with blue dashed lines. Red – Gaussian fits to each Na I line. We measure line contrasts of 0.373 ± 0.091% (D₂) and 0.508 ± 0.083% (D₁) and FWHMs of 0.619 ± 0.174 Å and 0.680 ± 0.128 Å, respectively. Lower panel: residuals of the Gaussian fit.