HARPS transmission light curve of HD 189733b. The three nights are averaged together with bins of 3 spectra. Left: relative fluxes are integrated in each spectrum over a 12 Å passband including the whole sodium Na i D doublet and compared to reference passbands in the same spectrum. The overall absorption depth is detected at 620 ppm with 7.8σ. Right: relative fluxes are integrated over two passbands of 0.75 Å around each sodium Na i D line. The absorption depth is detected at 3250 ppm with 9.8σ. Note the different vertical scales.