  In this paper we serendipitously identify X-ray cluster candidates using
XMM-Newton archival observations complemented by 5-band optical photometric
follow-up observations (r~23 mag) as part of the X-ray Identification (XID)
programme. Our sample covers an area of ~2.1 sq. deg (15 XMM-Newton fields) and
comprises a total of 21 (19 serendipitous + 2 target) extended X-ray sources to
the limit f(0.5-2keV) ~ 6x10^{-15} cgs with a high probability (> 99.9%) of
being extended on the XMM-Newton images. Of the 21 cluster candidates 7 are
spectroscopically confirmed in the literature. Exploiting the optical data
available for these fields we discover that 68% of the X-ray cluster candidates
are associated with optical galaxy overdensities. We also attempt to constrain
the redshifts of our cluster candidates using photometric methods. We thus
construct the photometric redshift distribution of galaxies in the vicinity of
each X-ray selected cluster candidate and search for statistically significant
redshift peaks against that of the background distribution of field galaxies.
Comparison of the photometric with spectroscopic redshift estimates for the
confirmed clusters suggest that our simple method is robust out to z~0.5. For
clusters at higher z, deeper optical data are required to estimate reliable
photometric redshifts. Finally, using the sample of the 19 serendipitous X-ray
selected cluster candidates we estimate their surface density down to
f(0.5-2keV) ~ 6x10^{-15} cgs and find it to be in fair agreement with previous
and recent studies.
