Three of the four models of the transmission spectrum of WASP-121 b assuming chemical equilibrium, a metallicity of 20× solar, a temperature of 1500 (blue) or 2000 K, with (yellow) and without (green) TiO opacity, used for model injection and comparison. Inset panels: models with (right) and without (left) the contribution of TiO opacity near 517 and 622 nm, both atthe high native resolution of these models (light colour), as well as broadened to include the effects of the instrumental resolution and rigid body rotation assumed for the planet (dark colour). VO absorption bands are only evident when TiO opacity is removed (the bands that remain visible in the green and blue models near, for example, 550 and 575nm), which would otherwise be masked by much stronger TiO bands. The fourth model (at 3000 K) is not plotted here, but is shown separately in Fig. 12.