Light curves of SDSS J1001+5027 in the r band and 29 spectroscopic epochs. The large circles/squares with black borders represent LT-HCT-PS magnitudes, while the small semi-transparent circles and squares describe Gaia-ZTF magnitudes (see main text for details). The vertical dotted lines show 25 epochs of spectroscopic observations with LT/SPRAT, leading to ten pairs of AB spectra separated by the time delay of the system (green arrows and horizontal green bars). For these AB pairs, the r-band magnitude offsets are highlighted with vertical green bars. The dashed vertical lines show four epochs of auxiliary spectroscopy (see Appendix B).