Lyman-α line of HD 189733b as a function of velocity, in April 2010 (Lecavelier des Etangs et al. 2012). 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. No absorption signatures were detected in either the transit or post-transit observations. The bottom panel shows the error bars on the flux measured before the transit.