Absorption spectrum (black line) of HD 189733 b around the sodium doublet, combining the first half of the two transits in the planetary rest frame, corrected for the POLDs using synthetic stellar spectra. The gray line shows the masked part of the absorption spectrum, which we excluded from the Gaussian fit (in red). The horizontal dashed blue line indicates the white-light radius (R_p/R_★)², and the vertical lines show the position of the sodium lines in the planet rest frame.