H/He vs. H₂O mass fractions for the best-fit interior compositions (≤1σ) permitted by the masses and radii of TOI-776 b and c, assuming an Earth-like core, for two different pressure-temperature profiles with radiative-convective boundaries at 1 and 100 bar. The blue shaded region indicates possible compositions for TOI-776 b, and the red shaded region shows compositions for TOI-776 c. The darker red shaded area between the two corresponds to the range of possible compositions that could explain both planets. For TOI-776 b, the H₂ O mass fraction is constrained to be ≤73% and the H/He mass fraction is ≤0.52%. For TOI-776 c the upper limit for H/He is 1.2%. A purely H₂O planet would be consistent with this mass and radius, but we only show H₂ O mass fractions up to 90%.