Deviation from the best-fitting model (Δ_comb) as a function of C/O ratio, for atmospheric lapse rates of 333 K/dex (top panel) and 111 K/dex (bottom panel). The C/O ratio is derived from the molecular abundances according to Eq. (6). Since multiple combination of molecular abundances give approximately the same C/O ratio, here we plot curves ranging from a high methane abundance ([CO]/[CH₄] = 1, black curve) to low methane abundance ([CO]/[CH₄] ≥ 10², orange curve). The points are fit with fourth-order polynomials. Except for steep lapse rates with highly-abundant methane, our analysis suggests an oxygen-rich atmosphere for HD 179949 b.