The ratio of the He-rich star’s rotational period to the orbital period (P₁/P_orb, left panel) and the resulting BH spin (χ_BH, right panel) are plotted as functions of model number for different initial orbital periods (red: 0.2 days; green: 0.5 days; blue: 1.0 days). Two synchronization timescales are compared: the solid line represents τ_sync from Sciarini et al. (2024), while the dashed line corresponds to τ_sync from Hurley et al. (2002). Vertical lines indicate the point at which the He-rich star reaches central helium depletion.