Lyman-α line as a function of velocity, observed by Vidal-Madjar et al. (2003) before (black line) and during (blue line) the transit of HD 209458b. The striped gray zone corresponds to the region contaminated by the geocoronal emission. The absorption signatures detected in the blue and red wings of the line are delimited by vertical dotted lines. The bottom panel shows the error bars on the flux measured before the transit.