Synthetic atmospheric transmission spectra of HD 260655 b (left) and c (right). Top: models of secondary H₂O (cyan) and CO₂-dominated (green) atmospheres. Middle: fiducial models of H/He atmospheres with solar abundances and no haze (solid red lines) and with haze (blue). Bottom: models of H/He atmospheres with enhanced metallicity by a factor of 100. Estimated uncertainties are shown for the observation of four/one transits with JWST NIRISS-SOSS, NIRSpec-G395H, MIRI-LRS configurations (cyan/red circles with black error bars), and with HST WFC3-G102 and G141 scanning modes (cyan/red squares with gray error bars). The gray region denotes the wavelength range 0.95–1.7 µm for which the host star brightness is beyond the saturation limits.