Top panel: 0.5–10 keV XMM-Newton/EPIC-pn light curve using 1 ks bins, with Type I X-ray bursts removed. The dashed blue line shows the spline interpolation obtained with the UnivariateSpline function from scipy.interpolate. The blue band indicates the interval of the HST observation discussed in this paper (see Sect. 3.2 for details). Second panel: Pulse fractional amplitude for the first harmonic (black dots) and the second harmonic (red dots). Third and fourth panels: Phase residuals relative to a linear model and a model that includes a phase-flux correlation term, respectively. Note that, where not visible, the error bars are smaller than the data points.