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The globular cluster 47 Tuc exhibits a complex sub-giant branch (SGB) with afaint-SGB comprising only about the 10% of the cluster mass and a bright-SGBhosting at least two distinct populations.We present a spectroscopic analysisof 62 SGB stars including 21 faint-SGB stars. We thus provide the firstchemical analysis of the intriguing faint-SGB population and compare itsabundances with those of the dominant populations. We have inferred abundancesof Fe, representative light elements C, N, Na, and Al, {\alpha} elements Mg andSi for individual stars. Oxygen has been obtained by co-adding spectra of starson different sequences. In addition, we have analysed 12 stars along the twomain RGBs of 47 Tuc. Our principal results are: (i) star-to-star variations inC/N/Na among RGB and bright-SGB stars; (ii) substantial N and Na enhancementsfor the minor population corresponding to the faint-SGB; (iii) no highenrichment in C+N+O for faint-SGB stars. Specifically, the C+N+O of thefaint-SGB is a factor of 1.1 higher than the bright-SGB, which, consideringrandom (+-1.3) plus systematic errors (+-0.3), means that their C+N+O isconsistent within observational uncertainties. However, a small C+N+Oenrichment for the faint-SGB, similar to what predicted on theoretical ground,cannot be excluded. The N and Na enrichment of the faint-SGB qualitativelyagrees with this population possibly being He-enhanced, as suggested by theory.The iron abundance of the bright and faint-SGB is the same to a level of ~0.10dex, and no other significant difference for the analysed elements has beendetected.

Chemical abundances in the multiple sub-giant branch of 47 Tucanae: insights on its faint sub-giant branch component