HARPS transmission spectrum of HD 189733b around the sodium Na i D doublet. Upper panel: master-out spectrum (stack of the out-of-transit spectra) normalized to unity. A telluric spectrum is also shown with a vertical offset to be able to identify water telluric lines. Middle panel: overall transmission spectrum of the exoplanet atmosphere taking into account all the observing nights (light gray). The transmission spectrum is shown binned by 20× with black circles. The planetary radial-velocity correction is applied. We can easily see the core absorption of the two Na i D lines from the planetary atmosphere. We show a Gaussian fit to each Na i D lines (red). We measure line contrasts of 0.64 ± 0.07% (D2) and 0.40 ± 0.07% (D1) and FWHMs of 0.52 ± 0.08 Å. The transition wavelengths of the doublet in the planet rest frame are indicated with the blue dashed line. A net blueshift is measured from our fit with a value of 0.16 ± 0.04 Å. This blueshift may be due to wind in the upper layer of the atmosphere with a velocity of 8 ± 2 km s^-1 (see text). Lower panel: residuals to the Gaussian fit. The noisy part is centered on the wavelength of the two sodium lines where the stellar flux is the lowest.