Folded Swift and NICER light curve. The upper panel shows the light curve of Fig. 10 folded at the best-fitting period of 19.9 d. Two cycles are shown for visual clarity, and the distinction between Swift and NICER data points has been removed. The solid red line represents the sinusoidal modulation with 1σ uncertainties in its amplitude shown as dotted lines. In the lower panel, we show the predicted modulation from Doppler boosting (blue) as well as one possible realisation of the disc precession model (red) corresponding to a disc misalignment of i_disc = 20° and observer inclination i_obs = 45°.