Gaussian process correction. Right panel: Transmission spectrum centred on the He I triplet (in the planet rest frame) with the GP regression model overplotted and with the 1 σ uncertainty intervals (in blue) and the Gaussian+GP model (in red). Left panel: Final transmission spectrum after removing the GP model. The error intervals for the Gaussian fit were computed by displaying 1000 Gaussian fits within the 1 σ uncertainties of the derived parameters spanning the 16%-84% quantiles. Vertical black dotted lines indicate the position of the He I triplet.