The large donor star has a tear-drop shape because it is filling its Roche lobe, donating mass to two compact stars in a tight binary that correspond to a pair of neutron stars or black holes. If the mass-gaining binary were simply a single star, mass would flow steadily through one point, the L1 point. Because the mass gainer is a binary, we see interesting patterns of mass flow, executing a complex dance.
Sophie Schroeder, Niels Bohr Institute, and Morgan Macleod, Institute for Theory and Computation at CfA.