Evolution of the progenitor binary of V1309 Scorpii before merger

Context. It was recently demonstrated that the eruption of V1309 Sco was a result of a merger of the components of a cool contact binary. Aims. We computed a set of evolutionary models of the detached binaries with different initial parameters to compare it with pre-burst observations of V1309 Sco. Methods. The models are based on our recently developed evolutionary model of the formation of cool contact binaries. Results. The best agreement with observations was obtained for binaries with initial masses of 1.8–2.0 M and initial periods of 2.5–3.1 d. The evolution of these binaries consists of three phases: at first the binary is detached and both components lose mass and angular momentum through a magnetized wind. This takes almost two thirds of the total evolutionary lifetime. The remaining third is spent in a semi-detached configuration of the Algol-type, following the Roche-lobe overflow by the initially more massive component. When the other component leaves the main sequence and moves toward the giant branch, a contact configuration is formed for a short time, followed by the coalescence of both components.