Locations of the low-mass intrinsic S stars (triangles) in the HR diagram, compared with STAREVOL evolutionary tracks of the closest metallicity (as indicated on the top label) and of the labelled masses. In the large panels, the RGB and the core He-burning phases are depicted in grey, whereas the red dashed line corresponds to the AGB tracks. The black dashed line marks the predicted onset of TDU. In the insert, the different colours correspond to different pulse-interpulse cycles of the 1 M_⊙ model star, as labelled.