Four transits of the planet orbiting the star HD209458 were observed with theSTIS spectrograph on board HST. The wavelength domain (1180-1710A) includes HIas well as CI, CII, CIV, NV, OI, SI, SiII, SiIII and SiIV lines. During thetransits, absorptions are detected in HI, OI and CII (5+/-2%, 13+/-4.5% and7.5+/-3.5%, respectively). No absorptions are detected for other lines. The 5%mean absorption over the whole HI Lyman alpha line is consistent with theprevious detection at higher resolution (Vidal-Madjar et al. 2003). Theabsorption depths in OI and CII show that oxygen and carbon are present in theextended upper atmosphere of HD209458b. These species must be carried out up tothe Roche lobe and beyond, most likely in a state of hydrodynamic escape.
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