Hertzsprung-Russell diagrams of α-rich HeCB stars in the Kepler and K2 fields compared to metal-rich GC HeCB stars and model predictions. Top panel: Kepler and K2 HeCB stars with −0.55 < [Fe/H] <  − 0.45 compared to GC stars of similar metallicity. The median colour of HeCB stars in each GC is corrected to [Fe/H] = −0.50 based on model comparisons in the bottom panel. Bottom panel: MESA HeCB stellar model track and median locations of various masses and compositions compared to observed median values. All model T_eff values are shifted by −126 K to reach agreement with the observed Kepler median T_eff at the corresponding observed 0.8 M_⊙. Crosses of different colours mark median locations of HeCB stars of different compositions. Model based translations between mass difference and temperature difference are illustrated by black double-arrows. Coloured double arrows mark the corresponding observed mass differences between the Kepler and GC HeCB medians.