HRD of a sample of red giants in the Kepler field. The coloured star symbols highlight the location of the first three rHB candidates in Table 1, and the grey dots in the background correspond to the Kepler-APOGEE sample in Miglio et al. (2021). The blue and red lines represent the theoretical red giant evolutionary tracks (from the RGB phase until the first thermal pulse) of low-mass stars with two different chemical compositions: M = 0.65 M_⊙, [α/Fe] = 0.2, [Fe/H] = −1.00 (blue) and M = 0.75 M_⊙, [α/Fe] = 0, [Fe/H] = 0 (red). The green line is the evolutionary track for a 1.5 M_⊙ with solar composition, and the dashed red one is the red edge of the RRL-IS for the composition of the blue track (see Marconi et al. 2015). Solid orange and blue circles correspond to our rHB and RC reference models, with a central He mass fraction Y_c ≃ 0.27.