Properties of discontinuous and nova–amplified mass transfer in CVs

We investigate the effects of discontinuous mass loss in recurrent outburst events on the long–term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. The Bondi–Hoyle accretion model is used to parameterize FAML in terms of the expansion velocity vexp of the nova envelope at the location of the secondary; we find that small vexp causes strong FAML. Numerical calculations of CV evolution over a wide range of parameters demonstrate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to mass transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model. FAML is a consequential angular momentum loss which amplifies the mass transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given vexp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass transfer amplitude induced by individual nova outbursts is proportional to it. In sequences calculated with nova model parameters taken from Prialnik & Kovetz (1995) FAML amplification is negligible, but the outburst amplitude in systems below the period gap with a white dwarf mass≃ 0.6M⊙ is larger than a factor of 10. The mass transfer rate in such systems is smaller than 10M⊙/yr for ≃ 0.5 Myr (≃ 10% of the nova cycle) after the outburst. This offers an explanation for intrinsically unusually faint CVs below the period gap.