Left: best-fit transit depths to HST data around the Na I lines, as reported in Table 3. Right: divide-oot detrended and phase-folded HST light curves (see Sect. 2.2.2) for the 5818–5887 Å (blue crosses), 5887–5899 Å (red dots), and 5899–5968 Å (dodger blue pluses) passbands, along with best-fit transit models (continuous coloured lines). The detrended out-of-transit data are not represented, as they are identical to 1 by definition of the divide-oot method (Berta et al. 2012). Bottom panel: transit model differences with respect to that of the central band. The slightly different shape between the two side bands is due to differential limb darkening.