We present a photometric and spectroscopic study of multiple populations along the asymptotic-giant branch ( AGB ) of the intermediate-metallicity globular clusters ( GCs ) NGC 2808 and NGC 6121 ( M 4 ) .
Chemical abundances of O , Na , Mg , Al , Si , Ca , Sc , Ti , V , Cr , Fe , Co , Ni , Zn , Y , and Ce in AGB stars from high-resolution FLAMES + UVES @ VLT spectra are reported for both clusters .
Our spectroscopic results have been combined with multi-wavelength photometry from the HST UV survey of Galactic GCs and ground-based photometry , plus proper motions derived by combining stellar positions from ground-based images and Gaia DR1 .
Our analysis reveals that the AGBs of both clusters host multiple populations with different chemical composition .

In M 4 we have identified two main populations of stars with different Na/O content , lying on distinct AGBs in the m _ { F 438 W } vs . C _ { F 275 W,F 336 W,F 438 W } and the V vs . C _ { U,B,I } pseudo-CMDs .

In the more massive and complex GC NGC 2808 three groups of stars with different chemical abundances occupy different locations on the so-called ” chromosome map ” photometric diagram .
The spectroscopic+photometric comparison of stellar populations along the AGB and the red giants of this GC suggests that the AGB hosts stellar populations with a range in helium abundances spanning from primordial up to high contents of Y \sim 0.32 .
On the other hand , from our dataset , there is no evidence for stars with extreme helium abundance ( Y \sim 0.38 ) on the AGB , suggesting that the most He-rich stars of NGC 2808 do not reach this phase .