2000ApJ...529..946P

The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a secondary that is much more luminous and hotter than is appropriate for a low-mass subgiant. We present detailed binary-evolution calculations which demonstrate that the present evolutionary state of Cyg X-2 can be understood if the secondary had an initial mass of around 3.5 M_☉ and started to transfer mass near the end of its main-sequence phase (or, somewhat less likely, just after leaving the main sequence, as recently suggested independently by A. R. King & H. Ritter). Most of the mass of the secondary must have been ejected from the system during an earlier rapid mass transfer phase. In the present phase, the secondary has a mass of around 0.5 M_☉ with a nondegenerate helium core. It is burning hydrogen in a shell, and mass transfer is driven by the advancement of the burning shell. Cyg X-2 therefore is related to a previously little studied class of intermediate-mass X-ray binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray binaries presently classified as low-mass X-ray binaries may be descendants of IMXBs and discuss some of the implications.