a) and b) Colour–magnitude diagrams for RGB stars with high–resolution spectroscopy of the Hercules dSph galaxy. • indicate RGB stars more metal-poor than  [Fe/H]  = −2.7. ° represents RGB stars more metal-rich than, or equal to,  [Fe/H]  = −2.7. The filled square indicates the most metal-rich RGB star at  [Fe/H]  = −2.0. The solid and dashed lines represent isochrones with  [Fe/H]  = −2.31 and  −2.14 dex, respectively, by VandenBerg et al. (2006) with colour transformations by Clem et al. (2004). c) Colour–magnitude diagram for the RGB stars in Adén et al. (2009a) with  [M/H] _{phot,   new} as determined in Sect. 5.4. • indicate RGB stars more metal-poor than  [M/H]  = −2.8. ° indicate RGB stars more metal-rich than  [M/H]  = −2.8. Isochrones as in b). The error bars on the right hand side in each figure represent the typical error in (v − y)₀.