Optical novae: the major class of supersoft X-ray sources in M 31 and M 33

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 one 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 to 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. This could indicate a larger rate of SSS among recurrent novae than among classical novae, or that the percentage of recurrent novae in M 31 has been strongly underestimated.