Left panel: position of the 152 stars in the log g − T_eff diagram. The samples are indicated by different colors and the common group includes the stars present in two or three samples. The error bars used for individual stars are the statistical uncertainties returned by the fitting procedure. The ZAEHB, TAEHB and the evolutionary tracks are as in Fig. 5. Right panel: same as the left panel but with the logarithmic helium abundance illustrated by the size of each circles, where super-solar and sub-solar abundances are represented by open and filled circles respectively. The circle size for a solar abundance is shown as an indication.