Bandpass-dependent geometric albedos. Left panel: examples of geometric albedo spectra assuming stellar abundances of H₂O (X_H2O = 5.36 × 10⁻⁴) and/or Na (X_Na = 2.19 × 10⁻⁶). The corresponding bandpass-integrated CHEOPS geometric albedos for H₂O and Na, H₂O only, and Na only are A_g = 0.11, 0.41, and 0.12, respectively. The bandpass filters of the CHEOPS, MOST, and TESS space telescopes are overlaid, as are the normalised spectral energy distribution of the HD 209458 star (where the fluxes have been divided by their maximum value such that they have no physical units). Right panel: modelled bandpass-integrated geometric albedos for MOST, CHEOPS, and TESS bandpasses assuming a stellar abundance of H₂O, but with varying volume mixing ratios of Na. The vertical dotted line corresponds to X_Na = 2.19 × 10⁻⁶, the Na abundance of the star. The horizontal line with the associated shaded region depicts the A_g = 0.096 ± 0.016 measurement reported in this study, and the MOST and predicted TESS 3σ upper limits are added as horizontal lines. All calculations assume a temperature of 1500 K; calculations with 1400 K produce a very similar outcome (not shown).