Colour-intensity diagrams of J1109 extracted from the most highly sampled g_S and i_S band NTT/ULTRACAM time series. Grey dots represent data points from the original time series, while black squares represent rebinned data points. All intensities have been corrected for extinction effects (see text for details). The results of the correlation tests performed on the rebinned data points are reported in the panels. The top panels display the kernel density estimation smoothing curves for the distribution of the colour values.