Left: optical B − I, V CMD showing the location of very metal-poor RRLs. The solid lines display predicted ZAHBs at different chemical compositions (see labelled values), while the dashed lines the central helium exhaustion for the same chemical compositions (Pietrinferni et al. 2021). Squares and triangles mark different stellar masses along the ZAHB and end-of-helium. The almost vertical lines display the blue (hot) and the red (cool) edge of the predicted RRL instability strip for Z = 1 × 10⁻⁵ specifically computed for this investigation (see text for more details). Right: same as the left, but for the optical-NIR (V − K, K) CMD.