In this section, I address accreting white dwarfs, which experience intermittent or continuous near-surface hydrogen burning. Burning occurs intermittently in novae and more or less continuously in close-binary supersoft X-ray sources (CBSS). The physics and X-ray emission of novae is more extensively addressed by Krautter (this volume). Continuous burning of accreted matter is of paramount interest as a means to drive a white dwarf above the Chandrasekhar mass limit into a SN Ia event [26,82].
The observationally defined "supersoft" X-ray sources (SSS) represent a mixed bag. They include single hot stars as the PG1159 stars, and also white dwarfs, which have experienced a late helium shell flash and become EUV sources in a "borne-again" scenario [33,64]. In binary stars, on the other hand, surface hydrogen burning on the white dwarf is not a singular phenomenon, but rather is rekindled over again or kept going by continued accretion. This subclass includes classical novae, recurrent novae, symbiotic novae, and CBSS.
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