Left panel: K_p map for the CO signal of WASP-18 b (top) and the corresponding CCF at the maximum S/N (bottom). The crossing of the dashed white lines indicates the location of the maximum S/N. In the bottom plot, the vertical dashed line and shaded region mark the RV peak and its 1σ confidence interval, respectively. Middle and right panels: same as the left panel, but for H₂O and OH, respectively.