Locations of the Tc-rich M stars AA Cam, OP Her, and RX Lac (filled triangles) in the HR diagram. The HR diagrams also include STAREVOL evolutionary tracks corresponding to the closest available metallicities ([Fe/H] = –0.5 or –1.0) with [O/Fe] = 0 and 0.4, hence AA Cam and OP Her appear in the left sub-panels, while RX Lac is shown in the right sub-panels. The dashed black line marks the predicted onset of the TDU, that is, the lowest stellar luminosity following the first occurrence of a TDU episode. For comparison, Tc-rich S stars (empty triangles) from Shetye et al. (2021) and Tc-poor S stars (empty squares) from Shetye et al. (2018) with metallicities around [Fe/H] = –0.5 are included.