Transmission spectrum of WASP-49b in the region of the sodium Na i D doublet as observed with HARPS. Upper panel: master-out spectrum and a reference telluric spectrum, for the first night, in the solar system barycentric reference frame. Middle panel: transmission spectrum of the exoplanet atmosphere (light gray), also binned by 20× in black circles. We show a Gaussian fit to each sodium line (red) with contrast of 1.99 ± 0.49% and 1.83 ± 0.65%, and FWHM of 0.42 ± 0.1 Å and 0.22 ± 0.08 Å for the D₂ and D₁ line, respectively. The rest-frame transition wavelengths are shown with the blue dashed line. Lower panel: Residuals to the Gaussian fit.