MULTIPLE HYDROGEN FLASHES ON ACCRETING LOW-MASS WHITE-DWARFS - NOVAE AND DAO STARS

We have computed 10 successive nova eruptions on two different mass white dwarfs as part of a study to determine the long-term evolution of cataclysmic binaries. The evolution of two low-mass white dwarfs, accreting hydrogen-rich matter at a rate of 10(-9) M. yr-1-typical of close binary systems-was calculated by means of a hydrodynamic code. The code includes algorithms for diffusion and mass-loss, and is capable of computing continuously several cycles, by adjusting the time steps to the prevailing evolutionary time scale. A 0.6 M. white dwarf, composed of carbon and oxygen, and a 0.4 M. helium white dwarf are considered, both of low intrinsic luminosity. They are found to undergo periodic eruptions, which, for the CO white dwarf become regular only after about five cycles. The outbursts are similar in appearance to those of classical novae, although on a largely stretched time scale. The high-luminosity (''on'') phase lasts for approximately 200 yr for both stars. The accretion (the '' off '' phase) continues for 4 x 10(5) yr and 9 x 10(5) yr, for the 0.6 and 0.4 M. WDs, respectively. Both stars lose mass following the outburst. While the more massive white dwarf ejects the entire accreted mass, the smaller, He WD retains some of the accreted material which is added to the core after being burnt into helium. During the accretion phase these objects should look and behave like dwarf novae. During the several thousand years after mass ejection ends these stars should evolve in appearance from nitrogen-rich PG 1159 objects (like PG 1144+005) to DAO and perhaps DB-like stars.