Left panel: NIR two-color plot [J − H] vs. [H − K]. The red point represents the NIR colors at MJD = 57 662 (see text). Continuous line: locus of T Tauri stars (Meyer et al. 1997). Dotted line: unreddened main-sequence. Dashed line: linear fit through the data. Arrow: extinction vector corresponding to A_V = 3 mag (reddening law of Cardelli et al. 1989). The average error is indicated with a cross. Middle panel: optical two-color plot [V − R] vs. [R − I]. Arrow: extinction vector corresponding to A_V = 1 mag. Dotted line: unreddened main-sequence. Red and dark-red large dots: our data taken before and after June 2017. The green dot represents the colors of 2015 June 22 (AAVSO database). Black data: literature/AAVSO data with the same symbols as in Fig. 1. Right panel: [V ] vs. [V − I]. Symbols and colors as in the middle panel.