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 \approx 23 mag ) as part of the X-ray Identification ( XID ) programme .
Our sample covers an area of \approx 2.1 deg ^ { 2 } ( 15 XMM-Newton fields ) and comprises a total of 21 ( 19 serendipitous + 2 target ) extended X-ray sources to the limit f _ { x } ( 0.5 - 2 keV ) \approx 6 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } , 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 \ga 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 \approx 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 _ { x } ( 0.5 - 2 keV ) \approx 6 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } and find it to be in fair agreement with previous and recent studies .