Example of a M_1,  i = 10.2 M_⊙, q_i = 0.4, and a_i = 12.4 R_⊙ binary system forming a contact binary during the MS of the primary (Case A) because of the thermal expansion of the secondary (accretor) star. Panel a shows an HRD with the evolutionary tracks of the primary and secondary stars (solid lines). The dashed black lines show the evolutionary tracks of single stars with the same initial masses as the binary components and initial rotation rates of Ω/Ω_c = 0.25. Panel b shows the evolution of the radius R (solid lines) and Roche lobe radius R_RL (dashed lines) of both components. The timescales governing the evolution of the binary and the mass-transfer rate (black dashed line) are shown in panel c as a function of the decreasing primary star mass. The secondary’s nuclear (τ_nuc,  2) and thermal (τ_KH,  2) timescales are shown with a dashed green and pink line, respectively. The expansion (τ_R/Ṙ,2) and mass-transfer (τ_trans) timescales are shown with a solid blue and orange line, respectively.