Time series of the S_MWO data (black dots). The best sine fit of the S_MWO data with one period is indicated by the solid red line, and the solid blue and green lines indicate the best sine fit with two and three periods, respectively. Top: All data. Bottom: Zoom into the top panel for the most recent decade. The F_X measurements from set III (red filled circles) with their errors are also plotted. In both panels, the magenta diamonds mark the data points of the minimum S_MWO value in each cycle.