Luminous supersoft X-ray sources were discovered with the Einstein observatory and have been established as an important new class of X-ray binaries on the basis of observations with the Roentgen Satellite (ROSAT). They have extremely soft spectra (equivalent blackbody temperatures of ∼15-80 eV) and are highly luminous (bolometric luminosities of 10 36 -10 38 erg s −1 ). Correcting for the heavy extinction of soft X rays by interstellar neutral hydrogen, their numbers in the disks of ordinary spiral galaxies like our own and M31 are estimated to be of the order of 10 3 . Their observed characteristics are consistent with those of white dwarfs, which are steadily or cyclically burning hydrogen-rich matter accreted onto the surface at a rate of order 10 −7 M year −1 . The required high accretion rates can be supplied by mass transfer on a thermal time scale (10 6 -10 7 years) from close companion stars that are more massive than the white dwarf accretor, typically 1.3-2.5 M . Steady burning can also occur in a post-nova stage, but for shorter time scales, and it has been observed in a few classical novae and symbiotic novae. A few supersoft sources have been found to be recurrent transients. They are possibly connected with very massive white dwarfs accreting at high rates. Luminous supersoft sources may make a considerable contribution to the Type Ia supernova rate in spiral and irregular galaxies.

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Abstract. We present a catalogue of 517 discrete X-ray sources in a 6• ×6• field covering the Small Magellanic Cloud (SMC). The catalogue was derived from the pointed ROSAT PSPC observations performed between October 1991 and May 1994 and is complementary to the Large Magellanic Cloud (LMC) catalogue published by . We followed the same identification scheme and used, among other information, X-ray hardness ratios and spatial extent to classify unknown sources as candidates for active galactic nuclei (AGN), foreground stars, supernova remnants (SNRs), supersoft sources (SSSs) and X-ray binaries. For 158 sources a likely source type is given, from which 46 sources are suggested as background AGN (including candidates resulting from a comparison of X-ray and radio images). Nearly all of the X-ray binaries known in the SMC were detected in ROSAT PSPC observations; most of them with luminosities below 10 36 erg s −1 suggesting that the fraction of high luminosity X-ray binary systems in the Magellanic Clouds (MCs) is not significantly larger than in our galaxy. Seventeen X-ray sources are associated with SNRs found in earlier work and we suggest here two additional extended sources as SNR candidates. Three very soft sources are newly classified as SSSs from which one is identified with the symbiotic star LIN 358 in the SMC.

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