Fig. 2a and b. a and indicate radii for subsequent evolutionary stages of the 20  primary as tabulated in Schaller et al. 1992. From the mass of the primary at the tabulated point one may calculate the semi-major axis of a binary with a 1  secondary in which the primary fills its Roche lobe, and from this the semi-major axis , shown as a solid line, of the binary at the beginning of its evolution. For each tabulated evolutionary stage we calculate the minimum semi-major axis at which the core of the primary survives the spiral-in of a 1  companion, and from this the minimum semi-major axis , indicated with the dashed line, of the binary at the beginning of its evolution. Primaries at the evolutionary stages marked with a cannot fill their Roche lobe for the first time at that stage but will reach their Roche lobe at an earlier point in their evolution. A low mass X-ray binary is formed when for evolutionary stages indicated with . b as a for a 60  primary