Lyman-α line of HD 189733b as a function of velocity, in September 2011 (Lecavelier des Etangs et al. 2012; Bourrier et al. 2013). The spectrum measured before the planet transit is displayed as a black line, to be compared with the spectra observed during (blue line, top panel) and after (blue line, bottom panel) the transit. The striped gray zone corresponds to the region contaminated by the geocoronal emission. The absorption signatures detected in the blue wing of the line during the transit and the post-transit and in the red wing during the transit are delimited by vertical dotted lines. The bottom panel shows the error bars on the flux measured before the transit.