Photometry of GJ 3942 including normalized data from APACHE (panel a), magnitude differences (GJ 3942 and calibration stars) of EXORAP filters B (panel b), V (panel c), R (panel d), and I (panel e), and visual magnitudes from the WASP (panel f) and Hipparcos catalogs (panel g). The red lines on the lower left show the rms of the nightly averages (panels a, f, g) and the RV rms of the calibration stars (b to e) as proxies for the uncertainties of the measurements. We note that APACHE was used during seasons 1 and 2 of the HARPS-N observations, EXORAP from season 2 to 4 and beyond, and WASP and Hipparcos data are from around 2007 and 1991, respectively.