1999MNRAS.309..253K

We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M₂≃0.5-0.7M_☉) and yet a large radius (R₂≃7R_☉) and high luminosity (L₂≃150L_☉). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the ∼3M_☉ transferred from the donor (initial mass M_2i∼3.6M_☉) on its thermal time-scale ∼10⁶yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super-Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio q_iM_2iM₁ in a narrow critical range near q_i≃2.6. Smaller q_i lead to long-period systems with the former donor near the Hayashi line, and larger q_i to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries.