Supersoft X-ray sources

More than 100 supersoft X-ray sources (SSS) are reported in similar to 20 external galaxies, the Magellanic Clouds (MCs) and our Galaxy. The effective temperatures of the brighter SSS are similar to 20-100 eV. SSS with luminosities below approximate to 3 x 10(38) erg s(-1) are consistent with accreting white dwarfs (WDs) with steady nuclear burning or post-novae. Optical identifications exist for SSS in our Galaxy and the MCs (including orbital period determinations) and for SSS in M31 (with novae and symbiotic stars, SySs). High resolution X-ray spectra of the brightest SSS in our Galaxy and the MCs reveal the existence of spectral features due to high gravity WDs. Timing studies in X-rays (combined with the optical) of the stable nuclear burning phase in steady nuclear burning sources and in post-novae allow to constrain the mass accretion rate onto and the mass of the nuclear burning WD. The nature of a few SSS with luminosities greater than or similar to 10(39) erg s(-1) remains unclear. (c) 2006 Published by Elsevier Ltd on behalf of COSPAR.