We discuss the formation of a common envelope system following dynamically unstable mass transfer in a close binary, and the subsequent dynamical evolution and final fate of the envelope. We base our discussion on new three-dimensional SPH calculations that we have performed for a close binary system containing a $4\,M_\odot$ red giant with a $0.7\,M_\odot$ main-sequence star companion. The initial parameters are chosen to model the formation of a system resembling V~471~Tau, a typical progenitor of a cataclysmic variable binary. In our highest-resolution calculation, we find evidence for a corotating region of gas around the central binary. This is in agreement with the theoretical model proposed by Meyer \& Meyer-Hofmeister (1979) for the evolution of common envelope systems, in which this central corotating region is coupled to the envelope through viscous angular momentum transport only. We also find evidence that the envelope is convectively unstable, in which case the viscous dissipation time could be as short as $\sim100$ dynamical times, leading to rapid ejection of the envelope. For V~471~Tau, our results, and the observed parameters of the system, are entirely consistent with rapid envelope ejection on a timescale $\sim1\,$yr and an efficiency parameter $α_{CE}\simeq1$.

uses AAS latex macros v4, 36 pages with figures, submitted to ApJ, complete postscript also available at http://ensor.mit.edu/~rasio/papers