Thermal stability and nova cycles in permanent superhump systems

Archival data on permanent superhump systems are compiled to test the therm al stability of their accretion discs. We find that their discs are almost certainly thermally stable as expected. This result confirms Osaki's sugges tion that permanent superhump systems form a new subclass of cataclysmic va riables (CVs), with relatively short orbital periods and high mass-transfer rates. We note that if the high accretion rates estimated in permanent sup erhump systems represent their mean secular values, then their mass-transfe r rates cannot be explained by gravitational radiation, therefore, either m agnetic braking should be extrapolated to systems below the period gap or t hey must have mass-transfer cycles. Alternatively, a new mechanism that rem oves angular momentum from CVs below the gap should be invoked. We suggest applying the nova cycle scenarios offered for systems above the period gap to the short orbital period CVs. Permanent superhumps have been observed in the two nonmagnetic ex-novae with binary periods below the gap. Their post-nova magnitudes are brighter than their pre-outburst values. In one case (V1974 Cyg) it has been demonstrated that the pre-nova should hav e been a regular SU UMa system. Thus, it is the first nova whose accretion disc was observed to change its thermal stability. If the superhumps in thi s system indicate persistent high mass-transfer rates rather than a tempora ry change induced by irradiation from the hot post-nova white dwarf, it is the first direct evidence for mass-transfer cycles in CVs. The proposed cyc les are driven by the nova eruption.