From top to bottom: light curves taken in 2011–2015 obtained before the long-term variation correction in the pre-whitening stage (see Sect. 3.1). The y-axis is in part-per-thousand. The removed sections of the light curves are shown in gray. Left column panels: represent the light curve as a function of time. The long-term variation is well observed on the data binned at two-hour intervals (red). Right column panels: represent these flux phase folded at the stellar rotation period of 38.8 days (binnedinto 30-min intervals). We note the different scale on the y-axis of the 2015 plot on the bottom.