time-series analysis for HD 15337. Top: raw time-series and low-pass filter shown as the green line. Blue points represent the data obtained before the change of fibre on June 1, 2015, and orange points show the data obtained afterwards. Middle: residual time-series after removing the contribution of the low-pass filter. We refer to this time-series as the high-frequency in the rest of the paper. Bottom: periodogram of the residuals with the 0.1% p−value shown as the horizontal red line. A significant peak appears at 36.5 days corresponding to the stellar rotation period of HD 15337.