Carbon measurements for the observed stars. Left panel: behavior of [C/Fe] (top) and ¹²C/¹³C ratio (bottom) as a function of [Fe/H] for the metal-poor (filled red circles) and metal-rich (filled blue squares) components of Liller 1. The error bars in the bottom right corners represent the typical error associated with the measurements. Right panel: ¹²CO 3−0 and 6−3 (top) and ¹³CO 4−1 and 6−3 (bottom) roto-vibration molecular bandheads in the H band, as observed in three metal-poor stars with similar stellar parameters, iron abundances, and ¹²C/¹³C isotopic ratios, but different carbon abundances.