Top: HRD of HB evolutionary models covering a broad range of stellar masses (M/M_⊙ = 0.48−0.90) and the same initial chemical composition (Z = 0.01, Y = 0.259). The grey area outlines the region between ZAHB (faint envelope) and central helium exhaustion (bright envelope). The green lines display HB models evolving as AGB-manqué, black lines the post early-AGB models, and purple lines the thermal pulsing AGB models (see text for details). The almost vertical blue and red solid lines indicate the hot and cool edges of the Cepheid instability strip. The minimum stellar mass (in solar units) crossing the instability strip is labelled in black. The black dashed lines show two iso-periodic lines for 5 and 20 days. Middle: same as the top panel, but for stellar masses ranging from M/M_⊙ = 0.4912 to 0.80 and for a metal-intermediate chemical composition (Z = 0.001, Y = 0.246). Bottom: same as the top panel, but for stellar masses ranging from M/M_⊙ = 0.5035 to 0.70 and for a metal-poor chemical composition (Z = 0.0001, Y = 0.245).