Delays in units of days of the dip arrival times plotted versus the corresponding cycle number; the Chandra point is indicated with a blue open square. Left panel: trial period of 24.52 d and epoch 50 086 MJD; the solid line represents the linear ephemeris shown in Eq. (1). Right panel: trial period of 24.5277 d and epoch 50 086.72 MJD. The solid line and the dashed curve indicate the linear ephemeris shown in Eq. (2) and the quadratic ephemeris of Eq. (3), respectively.