2008). The stellar parameters, reported in Table \[cncorrected\], were adopted from the detailed analyses by Zoccali et al. (2006) and Lecureur et al. (2007) for 43 bulge field red giants. Another 13 field red clump stars were analysed by Hill et al. (2011) based on both the UVES and the GIRAFFE spectra. We adopted the parameters derived from the UVES spectra, which were not given in Hill et al. (2011) but already reported in Barbuy et al. (2013).

C, N, O abundances and blending CN lines
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The 6362.339 [Å]{} line is blended with the CN lines reported in Table \[cnlines\], corresponding to the laboratory measurements by Davis & Phillips (1963). We initially adopted the C, N, and O abundances derived in Lecureur et al. (2007). Based on the observed profile of the 6362 [Å]{} line, however, we realized that in some cases the CN blend was overestimated, producing a spurious asymmetry in the red wing of the 6362 [Å]{} line. In these cases (16 stars marked with \*\* in Table \[cncorrected\], we recomputed C, N, and O abundances. We finally decided to recompute C, N, and O for all sample stars, given the influence of the CN blend in 