Short-term Fourier transform of the dataset used in Sect. 3.1. A 10.7-12.1-year-long cycle is seen throughout the whole plot, and in the more recent half of the light curve, a 7.3-8.2-year-long period also appears. Features longer than 27 years are suppressed on the plot to one-third of the power. The peak in this range originates from the length of the available data and the gap between 1960 and 1970.