  It was recently demonstrated that the eruption of V1309 Sco was a result of a
merger of the components of a cool contact binary. 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. The models are based on
our recently developed evolutionary model of the formation of cool contact
binaries. The best agreement with observations was obtained for binaries with
initial masses of 1.8-2.0 solar masses 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.
