GP correction. For each target, an example of GP correction is shown. Left panel: transmission spectrum centred on the He I triplet (in the planet rest frame) with overplotted the GP regression model, along with the 1σ uncertainty intervals, (in blue) and the Gaussian+GP model (in red). Right panel: final transmission spectrum after removing the GP model. Vertical black dotted lines indicate the position of the He I triplet. The spike in the right wing of the He I triplet in the GJ 3470 b transmission spectrum is a residual due to the OH emission line. The error intervals for the Gaussian fit were computed by displaying 1000 Gaussian fits within the 1σ uncertainties of the derived parameters, spanning the 16–84% quantiles.