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 \sim 10 ^ { -5 } M _ { \odot } and \sim 10 ^ { -6 } M _ { \odot } , 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 .