O–C diagram for WASP-12 b’s transit timing. The filled circles denote our observations and squares the literature ones with errors smaller than 40 s. The open symbols – circles and squares for our and literature data, respectively – mark points with timing errors greater than 40 s. The open triangles denote the amateur times included in this work. The postulated sinusoidal variation is sketched with a red line. Individual observing seasons, covered by our high-precision observations, are zoomed-in at the bottom. The literature data are taken from Hebb et al. (2009), Chan et al. (2011), Maciejewski et al. (2011), Sada et al. (2012), and Cowan et al. (2012).