Example of a grid of binary systems (initial orbital period versus initial primary mass) with Z = Z_⊙/ 50 and q_i = 1. Models that reached a point at which the difference between the surface and central helium abundance in one of the stars exceeds 0.2 are considered not to be evolving chemically homogeneously and their calculation is stopped (pink). The region in which the initial orbital period is small enough as to have L2 overflow at the ZAMS is marked in black, while those systems that reach L2 overflow during the main sequence are marked in green. Systems marked in blue successfully form double helium stars. Single hatching marks systems that experience contact during the main sequence, while doubly hatched systems are in an overcontact phase at the ZAMS.