GQ MUS (Nova Muscae 1983) was the first classical nova from which X-rays were detected during outburst1'2. Those Exosat observations were consistent with thermonuclear burning on the surface of a white dwarf emitting 10 37-1038 erg s-1 at (3.0-3.5) × 10 5 K or with shocked circumstellar material emitting 1035 erg s-1 by thermal bremsstrahlung at 107 K. Here we report the detection by the Rosat satellite3 of GQ Mus as a very soft black-body-like source. If the observed X-ray flux is being radiated at the Eddington luminosity (1038 erg s-1) from a one-solar-mass white dwarf, its effective temperature must be ∼3.5 × 105 K. We conclude that the white dwarf is burning hydrogen-rich material near its surface. GQ Mus is, however, the only one of 26 recent novae detected in the all-sky Rosat survey; this suggests that either most novae eject all their accreted material during outburst, or GQ Mus is now burning recently accreted material. GQ Mus appears identical to the supersoft X-ray sources CAL83, CAL87 and RX J0527.8-6954 (ref. 4), lending support to the suggestion that these sources are white dwarfs accreting and burning material from a companion 5.
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