Fig. 1. Lower limit to the initial semi-major axis at which the binary survives the spiral-in, as function of the mass of the primary. The upper (lower) dashed line gives the limit determined from the condition that the secondary star (helium core) is smaller than its Roche-lobe, The solid line gives the upper limit at which the primary reaches its Roche lobe at its maximum radius of . The secondary is assumed to be a 1  star