  We searched for X-ray counterparts of optical novae detected in M 31 and M
33. We combined an optical nova catalogue from the WeCAPP survey with optical
novae reported in the literature and correlated them with the most recent X-ray
catalogues from ROSAT, XMM-Newton and Chandra, and - in addition - searched for
nova correlations in archival data. We report 21 X-ray counterparts for novae
in M 31 - mostly identified as supersoft sources (SSS) by their hardness ratios
- and two in M 33. Our sample more than triples the number of known optical
novae with supersoft X-ray phase. Most of the counterparts are covered in
several observations allowing us to constrain their X-ray light curves.
Selected brighter sources were classified by their XMM-Newton EPIC spectra. We
use the well determined start time of the SSS state in two novae to estimate
the hydrogen mass ejected in the outburst to ~10^{-5}M_sun and ~10^{-6}M_sun,
respectively. The supersoft X-ray phase of at least 15% of the novae starts
within a year. At least one of the novae shows a SSS state lasting 6.1 years
after the optical outburst. Six of the SSSs turned on between 3 and 9 years
after the optical discovery of the outburst and may be interpreted as recurrent
novae. If confirmed, the detection of a delayed SSS phase turn-on may be used
as a new method to classify novae as recurrent. At the moment, the new method
yields a ratio of recurrent novae to classical novae of 0.3 which is in
agreement (within the errors) with previous works.
