Evolution of the orbital separation of tight binary systems composed of two solar mass stars. Upperpanel: evolutionary tracks for the systems shown in Fig. 2, which consist of one initially fast rotator and one initially slow rotator. The systems differ only in their initial orbital separations. Lower panel: orbital evolution in the first 0.5 Myr for four systems with starting initial separations of 0.05 AU. These systems differ in the initial rotation rates of their two stars, as indicated in the legend.