Optical light curve of AT 2000ch in Sloan r (orange), g (green), and i (purple) filters. Observations of amateur astronomers are included, shown as gray stars, as are unpublished data points from A. Pastorello (priv. comm.), shown as orange stars. Previously published photometry by Wagner et al. (2004), Pastorello et al. (2010), and Pastorello et al. (2013) (converted to the Sloan photometric system; see Sect. 4) is shown, as well as our photometric results from the PTF/ZTF survey observations. We only show the previously published data points in the same filter bands in which we added new observations. The vertical blue dotted lines indicate the dates of the observations that we collected in the radio range. Arrows at the top of the figure indicate the times of (black) previously known and newly detected or suspected (gray) eruptive episodes of re-brightening events based on Eq. (1) with their 1σ (3σ) uncertainty indicated by the gray (light gray) shaded areas.