and $G$ are determined by comparing the closely spaced measurements of the two datasets. The interval for comparison is adopted to be 50 days for the datasets D08 and W96 in Table \[tab\_lc\], which yields a good temporal overlap. For the [datasets D99b and P89]{} there is a short interval (2 years) of temporal overlap with the other datasets. Meanwhile, the sampling of these two datasets is poor. We thus increase the interval for comparison up to 100 days to obtain a sufficient number of nearly [contemporary]{} observations. Since we concentrate on long-term variations, the choice of the interval has no influence on our analysis. It is impossible to intercalibrate the H$\beta$ fluxes of our new observations with the other datasets, because there is no temporal overlap. We present the light curve of the H$\beta$ fluxes for the purpose of illustrating the variation behaviour of [the]{} H$\beta$ line. We do not use it for subsequent periodicity analysis.

For $V-$band photometric data, we first convert the magnitudes into flux densities by adopting the zero point $F_\lambda(V=0)=3.92\times10^{-9}\,{\rm erg~s^{-1}~cm^{-2}}\textrm{\AA}^{-1}$ (@Johnson1966). We then scale the flux densities to align with the 5100 [Å]{} flux densities by again applying a scale factor ($\varphi$) and flux 