Contour maps of the reduced χ² of the He(2³S) absorption for HD 189733 b (left panel) and GJ 3470 b (right panel) for an H/He ratio of 90/10. We note the different scales of temperatures and Ṁ. Dotted curves represent the best fits with filled circles denoting the constrained ranges for a confidence level of 95% (see Sect. 3.4). Over-plotted are also the curves and symbols for several H/He ratios, as labelled. The labels correspond to the hydrogen percentage, e.g. ‘90’ for an H/He of 90/10 and ‘95’ for H/He = 95/5 (see Sect. 4.4). The black dots represent the grid of the simulations.