We present new empirical estimates of the \Delta V _ { HB } ^ { bump } parameter for 15 Galactic globular clusters ( GGCs ) using accurate and homogeneous ground-based optical data .
Together with similar evaluations available in the literature , we ended up with a sample of 62 GGCs covering a very broad range in metal content ( –2.16 \leq [ M/H ] \leq –0.58 dex ) .
Adopting the homogeneous metallicity scale provided either by Kraft & Ivans ( 2004 ) or by Carretta et al .
( 2009 ) , we found that the observed \Delta V _ { HB } ^ { bump } parameters are larger than predicted .
In the metal-poor regime ( [ M/H ] \lesssim –1.7 , –1.6 dex ) 40 % of GCs show discrepancies of 2 \sigma ( \approx 0.40 mag ) or more .
Evolutionary models that account either for \alpha - and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations .
The outcome is the same if we use the new Solar heavy-element mixture .
The comparison between \alpha - and CNO-enhanced evolutionary models and observations in the Carretta et al .
metallicity scale also indicates that observed \Delta V _ { HB } ^ { bump } parameters , in the metal-rich regime ( [ M/H ] \geq 0 ) , might be systematically smaller than predicted .