Continuing activity in the old novae HR Del and RR Pic

Graphs of old nova ultraviolet luminosities against orbital inclination show that there is an inverse correlation between the two quantities. DQ Her and GK Per, which are thought to be magnetic systems with disrupted inner disks, appear to have low ultraviolet luminosities. Instead, HR Del has a considerably higher luminosity than other novae with similar inclination and RR Pic is brighter also. The question arises whether these novae have maintained a form of activity possibly involving a low level of nuclear burning on a low mass white dwarf for many decades after the outburst

Common Envelope Evolution Redux

Common envelopes form in dynamical time scale mass exchange, when the envelope of a donor star engulfs a much denser companion, and the core of the donor plus the dense companion star spiral inward through this dissipative envelope. As conceived by Paczynski and Ostriker, this process must be responsible for the creation of short-period binaries with degenerate components, and, indeed, it has proven capable of accounting for short-period binaries containing one white dwarf component. However, attempts to reconstruct the evolutionary histories of close double white dwarfs have proven more problematic, and point to the need for enhanced systemic mass loss, either during the close of the first, slow episode of mass transfer that produced the first white dwarf, or during the detached phase preceding the final, common envelope episode. The survival of long-period interacting binaries with massive white dwarfs, such as the recurrent novae T CrB and RS Oph, also presents interpretative difficulties for simple energetic treatments of common envelope evolution. Their existence implies that major terms are missing from usual formulations of the energy budget for common envelope evolution. The most plausible missing energy term is the energy released by recombination in the common envelope, and, indeed, a simple reformulation the energy budget explicitly including recombination resolves this issue.Comment: 25 pages, 6 figures. To appear in "Short Period Binary Stars", ed. E.F. Milone, D.A. Leahy, & D.W. Hobill (Springer