Transit timing residuals from the linear ephemeris. The black open and filled circles denote literature and new data, respectively. Each point represents a half-season average of individual observations, which are marked with gray dots. The continuous line sketches the quadratic ephemeris discussed in Sect. 3. The dashed line illustrates the periodic signal discussed in Sect. 4. The gray area shows the next observing season, in which new data are expected from which we hope to be able to distinguish between both models.