Panel a: Swift/XRT monitoring light curve in the 0.3–10 keV energy band. The three columns focus on epochs 2013, 2014, and 2016–2017. The red curve shows the light curve folded on a 66.9 d period plus a linear brightening trend; the blue dashed line only shows the linear trend. Panel b: measured values of the pulse period as a function of time. Diamonds indicate values taken with NuSTAR and circles values taken with XMM-Newton. The small square shows the period measured in observation 2017A with XMM-Newton, which did not provide a significant detection by itself. The red line is the best-fit model using the pulse profile as input; the blue dashed line a linear trend only. The model takes the secular spin-up as function of intrinsic luminosity and the orbital Doppler motion into account. For details see text. Panel c: residuals as data-minus-model for both models (red: pulse profile input, blue: linear trend only). Panel d: measured and predicted instantaneous change of the period (Ṗ). The XMM-Newton data are shown with gray circles and are not constraining due to the shorter exposure time of the XMM-Newton observations. The models were fitted without taking Ṗ into account. The good agreement between data and model is therefore an independent confirmation of the orbital ephemeris. The time axis is given in MJD.