Optical novae: the major class of supersoft X-ray sources in M 31 W. Pietsch, J. Fliri, M. J. Freyberg, J. Greiner, F. Haberl, A. Riffeser and G. Sala A&A, 454 3 (2006) 773 Published online: 17 July 2006 DOI: 10.1051/0004-6361:20053127e