Evolutionary tracks in the HR diagram of a close binary system with an initial primary mass of 27.60 M_⊙ and a secondary mass of 23.18 M_⊙. This binary is assumed to start its evolution in a circular orbit with P_orb = 2 days, in a fully conservative MT scenario throughout its entire lifetime. The evolution of the primary (secondary) star is represented in solid blue (orange) line. Different MT phases are plotted on top of each star: in green (grey) we show Case A (early Case B) MT. The dotted grey lines represent constant radii for 1, 10, and 100 R_⊙, while the dotted black line designates the evolution of the secondary star after the formation of the first compact object (GX 301–2) if the companion NS star was absent. The yellow star marks the location where the primary exploded as a SN. For comparison, the ZAMS corresponding to stars of the same composition is shown by the black dashed line.