Differential-reddening-corrected and proper-motion-selected CMD of all the stars detected within 20^′′ from the RB X-ray position (gray dots). The black circles and the large red star highlight the sources located within a distance of 1.5^′′ and the candidate optical counterpart to the RB, respectively. The red and blue lines (see also legend) are a 12 Gyr old isochrone (Bressan et al. 2012) with [M/H]=−0.3 (well reproducing the old subpopulation of Liller 1), and two 1–2 Gyr old isochrones with [M/H]=+0.3 (tracing the young and supersolar subpopulation), respectively.