Broadband (white) light curve for the transit of WASP-17b, fitted with a GP model described in Sect. 3.3. The best fitting transit light curve is shown in blue with the GP mean model given as solid red line. The residuals of both models are shown at the bottom, where they are shifted for clarity and their colours correspond to the models that they represent. The dark and light grey regions represent the 1 and 3σ predictions of the GP model, respectively. The green line shows the systematics model without the transit function.