HR diagram of intrinsic (filled triangles) S stars from Shetye et al. (2018, 2019), bitrinsic S stars (blue stars), and extrinsic (open squares) S stars from Shetye et al. (2018) along with the STAREVOL evolutionary tracks corresponding to the closest grid metallicity. The red giant branch is represented by the gray dots, whereas the red dashed line corresponds to the AGB tracks. The black dashed line marks the onset of TDU.