O–C diagram of transit times from photodynamical modelling of data set III and the predictions until 2020 (top). Calculated (C) are transit times from a linear ephemeris modelled at the transit times found by Dreizler & Ofir (2014). The residual plots (middle: Kepler-9b; bottom: Kepler-9c) show the 99.74% confidence interval of 1000 randomly chosen good models in comparison with the best model of data set III. From light to dark grey: modelling of data set I, II, and III. The Kepler transit times are derived by single transit modelling. The new transit time data points originate from the first analysis described in Sect. 2.3.