Spectral transmission of the He triplet at mid-transit for HD 189733 b and GJ 3470 b (we note the different y-axis scale). Data points and their respective error bars are shown in black (adapted from Salz et al. 2018 and Pallé et al. 2020, respectively). Wavelengths are given in vacuum. The best-fit simulations are shown with red curves. For HD 189733 b, the best fit corresponds to a temperature of 10 000 K, a mass-loss rate of 10⁹ g s⁻¹ and an H/He mole-fraction of 90/10. For GJ 3470 b, the best fit corresponds to a temperature of 6000 K, Ṁ = 3.2 × 10¹⁰ g s⁻¹ and an H/He of 90/10. Other models are described in Table 2. The positions of the helium lines are marked by vertical dotted (weak) and dot-dashed (strong) lines.