Representative binary from the SMT channel leading to a BBH merger at 10⁻⁴ Z_⊙ with M₁ = 32 M_⊙, M₂ = 27 M_⊙, and P = 1.9 days at ZAMS. The time series is divided into different scales to show details in the evolution on shorter timescales. The top row shows the mass evolution of the primary (blue) and secondary (red), while the bottom row shows their stellar radius evolution. The shaded regions are when RLO occurs, colored according to the donor star. The black solid line tracks the orbital separation, and the vertical dashed lines mark the times of core collapse for each component. Between 5.58 Myr and 5.59 Myr, the first mass transfer phase starts. After a fast thermal timescale interaction, the mass ratio flips, and the mass transfer continues on a nuclear timescale. At ∼7.68 Myr, the detached binary with an eccentricity of 0.001 is circularized and matched to the nearest CO-HMS_RLO grid model, causing the slight increase in mass, period, and radius. The steep drop afterward is caused by thermal timescale mass transfer. The small changes in radii at 6.2 Myr and 8.05 Myr are from readjustment after core hydrogen depletion in the primary and secondary, respectively. In the top row, the He-core (M_He) and CO-core (M_CO) masses are shown with dashed and dotted lines, respectively. This system is evolved with the same simulation setup as the population models, but the nearest-neighbor method (matching to the closest down-sampled precomputed track) is used instead of initial–final interpolation to show the evolution over time (for more details, see Fragos et al. 2023). The orange bars indicate the POSYDON step or grid that is used for the evolution of the binary.