Comparison of the observed CMD with isochrones. Left-hand panel: JWST CMD of NGC 6440 including only bona-fide member stars. The red solid curve is a 13 Gyr BaSTI isochrone reproducing a [α/Fe]= + 0.4 stellar population with a metallicity [Fe/H] = −0.6 and a He fraction Y = 0.257, representative of the FP. The blue curve is the same model but with a He fraction Y = 0.30. Right-hand panel: same as the left-hand panel but zoomed on the lower main sequence. The solid curve is the same as in the left-hand panel and reproduces the FP (O/Fe = +0.4), while the dashed and dotted curves are representative of two different SPs with a solar-scaled (O/Fe = +0.0) and (O/Fe = −0.4) oxygen abundance, respectively.