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The existence of star-to-star light-element abundance variations (multiplepopulations, MPs) in massive Galactic and extragalactic star clusters olderthan about 2 Gyr is by now well established. Photometry of red giant branch(RGB) stars has been and still is instrumental in enabling the detection andcharacterization of cluster MPs, through the appropriate choices of filters,colours and colour combinations, that are mainly sensitive to N and --to alesser degree-- C stellar surface abundances. An important issue not yetproperly addressed is that the translation of the observed widths of thecluster RGBs to abundance spreads must account for the effect of the firstdredge-up on the surface chemical patterns, hence on the spectral energydistributions of stars belonging to the various MPs. We have filled this gap bystudying theoretically the impact of the dredge-up on the predicted widths ofRGBs in clusters hosting MPs. We find that for a given initial range of Nabundances, the first dredge up reduces the predicted RGB widths in N-sensitivefilters compared to the case when its effect on the stellar spectral energydistributions is not accounted for. This reduction is a strong function of ageand has also a dependence on metallicity. The net effect is an underestimate ofthe initial N-abundance ranges from RGB photometry if the first dredge-up isnot accounted for in the modelling, and also the potential determination ofspurious trends of N-abundance spreads with age.

Photometric characterization of multiple populations in star clusters: The impact of the first dredge-up