Residuals to the orbital fit of (4337) Arecibo (blue dots). They are obtained from the average of single-observation residuals over each transit. The error bars are given by their standard deviation. In the top panel, the dashed grey line is not a model fit, but a simple overplotting of a sinusoid of the period derived by photometry, adjusted in amplitude (0.8 mas) and phase to the data. In the bottom panel, the same data are shown with the residuals predicted by the optimised binary model described in the text (red crosses).