Estimated geometric albedo of WASP-18 b as a function of the abundance of H⁻. The A _g values are computed based on the retrieval results in comparison to the passbands of CHEOPS and TESS. The H⁻ abundance that best matches our measurements reported in Section 6.3 lies around 2% of the expected chemical equilibrium value (100% on the x-axis), i.e. a reduction of 98%.