Upper panel: differences between the observed and modelled transit centre times of Kepler-423 (TTVs). Transit timing variations extracted from the short cadence data are plotted with red triangles, while long cadence TTVs are shown with blue circles. Middle panel: Lomb-Scargle periodogram of the TTV data in the 5–30 day period range. The two arrows mark the peak close to the stellar rotation period and its first harmonic. Lower panel: transit timing variation versus local transit slope. The straight blue line marks the linear fit to the data.