The minimum orbital period in thermal-timescale mass transfer

We show that the usual picture of supersoft X-ray binary evolution as driven by conservative thermal-timescale mass transfer cannot explain the short orbital periods of RX J0537.7-7034 (3.5 hr) and 1E 0035.4-7230 (4.1 hr). Non-conservative evolution may produce such periods, but requires very significant mass loss, and is highly constrained.