Detection of H₂O in the transmission spectrum of HD 189 733 b observed on July 11, 2015 (top panels) and the non detection on the night of July 30, 2015 (bottom panels). The total cross-correlation signal is shown as function of rest-frame velocity ν_rest and planet radial velocity semi-amplitude K_P. The measured S/N and corresponding significances are shown in the left and right panels, respectively. These are computed as explained in Sect. 3.3. The cross-correlation shown here is obtained with a model spectrum containing H₂O at VMR = 10⁻⁴ and CH₄ at VMR = 10⁻¹².