Left: Kiel diagram of HE 1518 illustrating several evolutionary tracks that intersect this region of the diagram. The black solid lines show post-merger evolutionary tracks of C/O+He white dwarf mergers with total masses of 0.7, 0.8, and 0.9 M_⊙ from Saio & Jeffery (2002). The orange dashed line represents the evolution of an initially 7.4 M_⊙ star stripped by a companion from Götberg et al. (2018), resulting in a helium star with a mass of approximately 1.9 M_⊙. The blue line shows the evolution of a star that experienced a very late thermal pulse (VLTP) and is contracting towards higher effective temperatures to enter the white dwarf cooling track from Miller Bertolami & Althaus (2006). The model shown has a mass of 0.51 M_⊙. The violet line corresponds to the evolution of a double helium white dwarf merger from Yu et al. (2021), in which both components have masses of 0.45 M_⊙. The light green region indicates the location of the He-ZAMS from Paczyński (1971) for helium stars with masses between 0.6 and ∼2.0 M_⊙ The arrows indicate the direction of evolution for the corresponding tracks. Right: Stellar evolution for helium stars with 0.7 (gold), 0.8 (blue), and 0.9 M_⊙ (black) for two different metallicities Z = 0.001 (solid line) and Z = 0.02 (dashed line; for more details see Sect. 8.1). The grey dots are the observed helium-rich hot subdwarfs in the SALT survey of hydrogen-deficient stars (Jeffery et al. 2026, preliminary results).