Systematics-corrected HST/WFC3 white-light phase curve of WASP-18b (blue points) compared to suite of GCMs (coloured curves). In black is the best-fit model used to parametrise the planet signal, with two sinusoidal components for the phase-curve variation. In cyan are the stellar ellipsoidal variations, that have been subtracted from the phase curve (see Sect. 3.2). GCMs shown are parametrised by 3 drag timescales indicated by line-styles and by 3 metallicities indicated by colours (−0.5, 0.0, and +0.5 relative to solar).