Radius variation (measured with R_p/R_∗ and indicated with yellow disks with error bars) of WASP-23 b in the seven optical to NIR passbands, compared to a synthetic transmission spectrum based on a planet-wide pressure-temperature profile (grey continuous line). The open red diamonds indicate the expected theoretical values, integrated over GROND’s filter curves. The latter are displayed as dotted lines at the origin of the graph.