  We used ultra-deep observations obtained with the NIRCam aboard the James
Webb Space Telescope to explore the stellar population of NGC 6440: a typical
massive, obscured and contaminated globular cluster formed and orbiting within
the Galactic bulge. Leveraging the exceptional capabilities of this camera, we
sampled the cluster down to ~5 magnitudes below the main-sequence turn-off in
the (mF115W , mF115W - mF200W ) colour-magnitude diagram. After carefully
accounting for differential extinction and contamination by field interlopers,
we find that the main sequence splits into two branches both above and below
the characteristic knee. By comparing the morphology of the colour-magnitude
diagram with a suitable set of isochrones, we argue that the upper
main-sequence bi-modality is likely due to the presence of a He-enriched
stellar population with a helium spread of DeltaY = 0.04. The lower
main-sequence bi-modality can be attributed to variations in the abundance of
water (i.e., oxygen) with Delta[O/Fe] ~ -0.4. This is the first evidence of
both helium and oxygen abundance variations in a globular cluster purely based
on JWST observations. These results open the window for future in-depth
investigations of the multiple population phenomenon in clusters located in the
Galactic bulge, which were previously unfeasible with near-UV observations, due
to prohibitive reddening and crowding conditions.
