Panel a: evolution of the He i D3 line equivalent width. Panel b: evolution of the excess equivalent width, W_ex, in the core of the He i D3 line converted into the flux measured at Earth (see also Fig. 7). Shaded pink area indicates the duration of an exoplanetary transit. Meaning of dashed lines from left to right: middle of the transit T₀ [HJD,UTC] = 2 456 109.26164; start of the rise phase; end of the rise phase. The 1σ error for the displayed data points is within the 4−6% range.