Double white dwarfs as progenitors of R Coronae Borealis stars and Type I supernovae

Tidal mass transfer in double degenerate systems is explored. The sequence of double white dwarfs divides naturally into three segments: (1) low-mass helium/helium pairs are unstable to dynamical time-scale mass transfer and probably coalesce to form helium-burning sdO stars; (2) in helium/carbon-oxygen pairs, mass transfer occurs on the time scale for gravitational radiation losses; the accreted helium is quickly ignited, and the accretor expands to dimensions characteristic of R CrB stars, engulfing its companion star; and (3) carbon-oxygen/carbon-oxygen pairs are again unstable to dynamical time-scale mass transfer and, since their total masses exceed the Chandrasekhar limit, are destined to become supernovae. Inactive lifetimes in these latter systems between creation and interaction can exceed 10 billion years. Birthrates of R CrB stars and Type I supernovae by evolution of double white dwarfs are in reasonable agreement with observational estimates.