To understand globular clusters ( GCs ) we need to comprehend how their formation process was able to produce their abundance distribution of light elements .
In particular , we seek to figure out which stars imprinted the peculiar chemical signature of GCs .
One of the best ways is to study the light-element anti-correlations in a large sample of GCs that are analysed homogeneously .
As part of our spectroscopic survey of GCs with FLAMES , we present here the results of our study of about 30 red giant member stars in the low-mass , low-metallicity Milky Way cluster NGC 6535 .
We measured the metallicity ( finding [ Fe/H ] = -1.95 , rms=0.04 dex in our homogeneous scale ) and other elements of the cluster and , in particular , we concentrate here on O and Na abundances .
These elements define the normal Na-O anti-correlation of classical GCs , making NGC 6535 perhaps the lowest mass cluster with a confirmed presence of multiple populations .
We updated the census of Galactic and extragalactic GCs for which a statement on the presence or absence of multiple populations can be made on the basis of high-resolution spectroscopy preferentially , or photometry and low-resolution spectroscopy otherwise ; we also discuss the importance of mass and age of the clusters as factors for multiple populations .