Turning on and off mass transfer in cataclysmic binaries.

The author presents a simple model for the mass transfer in nearly semi-detached binaries that takes the finite scale height of the stellar atmosphere of the lobe-filling component properly into account. Application of this model to cataclysmic binaries (CBs) yields the following results: For mass transfer rates M⪉ 10^-8M_sun;yr^-1 the lobe-filling component is actually underfilling its critical potential by about one to a few scale heights H_P. The time scale on which mass transfer turns on is of the order H_P/R₂ times the evolutionary time scale. If evolution is driven by magnetic braking the latter is of order 10⁸yr and mass transfer turns on over a time scale ≡10⁴yr.