  To investigate the transient nature of supersoft sources (SSSs) in M 31, we
compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and
the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton
observations. While 12 of the XMM-Newton sources were brighter than the
limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10
yr earlier. Five correlate with recent optical novae which explains why they
were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below
the ROSAT detection threshold. Nevertheless we found one correlation with a
ROSAT source, which had significantly larger fluxes than during the XMM-Newton
observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr
between the observations. Five were also classified as SSSs by Chandra. Of the
30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other
classifications are suggested. Of the remaining 16 sources one correlates with
an optical nova. Of the 42 Chandra very-soft sources five are classified as
XMM-Newton SSSs, while for 22 we suggest other classifications. Of the
remaining 15 sources, nine are classified as transient by Chandra, one of them
correlates with an optical nova. These findings underlined the high variability
of the sources of this class and the connection between SSSs and optical novae.
Only three sources, were detected by all three missions as SSSs. Thus they are
visible for more than a decade, despite their variability.
