peaks at 20.5$\pm$0.1 years. Here the uncertainty is determined from the uncertainty of the peak frequency given by the formula in @Horne1986 [see Equation (14) therein]. The LS periodogram essentially assumes a sinusoidal model for the data (@Scargle1983 [@VanderPlas2018]).

-   [*Phase dispersion minimization (PDM) analysis*]{}[[^5]]{}. The PDM method is widely used to search for non-sinusoidal periodicity by minimizing the dispersion of the phase-folded data sets (@Stellingwerf1978). The bottom right panel of Figure \[fig\_period\] plots the PDM periodogram, which is minimized at $20.5$ years, consistent with the period from the LS periodogram.

-   [*Sinusoidal fit*]{}. The sampling rate of the light curve is highly irregular and some portions have quite dense cadences. Besides, the light curve fluctuates strongly on time scales of weeks/months. To alleviate these two effects, we bin the light curve every 300 days, resulting in a total of 45 data points over a baseline of 43 years. The uncertainties of points in the newly binned light curve are assigned to be the standard deviations of each bin. The sinusoidal fitting to the binned light curve yields a period of $19.7\pm0.5$ years and sinusoidal amplitude of $(2.32\pm0.20)\times10^{-15}\,\rm erg~s^{-1}cm^{-2}\text{\AA}^{-1}$. The obtained period slightly depends on the adopted bin width, with 