O-C diagrams for planets b, c, and d (from top to bottom) of the Kepler-11 system; the black filled circles are the observed points fitted by a linear ephemeris, the blue open circles are the simulated points fitted by the same linear ephemeris of the observations. The simulated points are calculated from the solution K11-III (best simulation in Fig. 6) in Table 2. Residual plots, as the difference between observed and simulated central time (T_0,obs − T_0,sim), are in the lower panel of each O-C plot. The unit of measurement of the left O-C y-axis is days (d) and minutes (m) are on the right. The N in the abscissa identifies the transit number respect to the reference transit time of the ephemeris of each body (second column of Table 3).