For each QPE, we plot in the top panel the 0.2 − 2 keV background-subtracted QPE light curve, centered around the QPE peak and binned to 250 s, and in the lower panel the hardness ratio HR = CR(0.6 − 2)/CR(0.2 − 2), where CR(0.6 − 2) is the count rate in the 0.6 − 2 keV band and CR(0.2 − 2) is the total count rate in the 0.2 − 2 keV band. Each point is color-coded following the time evolution of the QPE in the panel above. The HR shows a counterclockwise evolution, as highlighted by the dashed arrow. All these quantities have been computed using the pn data only.