Left: HRD of HB models crossing the instability strip, calculated by assuming an analytical extension for the AGB evolutionary phases (black solid lines) and HB models with the same stellar mass with a full evolutionary computation (black dashed line). From top to bottom: theoretical predictions refer to metal-rich (M = 0.505 M_⊙), metal-intermediate (M = 0.505 M_⊙), and metal-poor (M = 0.520 M_⊙) models. The almost vertical blue and red lines display the edges of the instability strip. Right: same as the left panel, but the comparison is between standard He (black solid line) and He-enhanced (Y = 0.30) models (black dashed lines). See text for details.