  The globular cluster 47 Tuc exhibits a complex sub-giant branch (SGB) with a
faint-SGB comprising only about the 10% of the cluster mass and a bright-SGB
hosting at least two distinct populations.We present a spectroscopic analysis
of 62 SGB stars including 21 faint-SGB stars. We thus provide the first
chemical analysis of the intriguing faint-SGB population and compare its
abundances with those of the dominant populations. We have inferred abundances
of Fe, representative light elements C, N, Na, and Al, {\alpha} elements Mg and
Si for individual stars. Oxygen has been obtained by co-adding spectra of stars
on different sequences. In addition, we have analysed 12 stars along the two
main RGBs of 47 Tuc. Our principal results are: (i) star-to-star variations in
C/N/Na among RGB and bright-SGB stars; (ii) substantial N and Na enhancements
for the minor population corresponding to the faint-SGB; (iii) no high
enrichment in C+N+O for faint-SGB stars. Specifically, the C+N+O of the
faint-SGB is a factor of 1.1 higher than the bright-SGB, which, considering
random (+-1.3) plus systematic errors (+-0.3), means that their C+N+O is
consistent within observational uncertainties. However, a small C+N+O
enrichment for the faint-SGB, similar to what predicted on theoretical ground,
cannot be excluded. The N and Na enrichment of the faint-SGB qualitatively
agrees 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 ~0.10
dex, and no other significant difference for the analysed elements has been
detected.
