  In this paper we investigate the properties of low X-ray-to-optical flux
ratio sources detected in a wide area (2.5deg^2) shallow
(f(0.5-8keV)~10e-14cgs) XMM-Newton survey. We find a total of 26 sources (5% of
the total X-ray selected population) with log f_X/f_{opt}<-0.9 to the above
flux limit. Optical spectroscopy is available for 20 of these low
X-ray-to-optical flux ratio objects. Most of them are found to be associated
with Galactic stars (total of 8) and broad line AGNs (total of 8).We also find
two sources with optical spectra showing absorption and/or narrow emission
lines and X-ray/optical properties suggesting AGN activity. Another two sources
are found to be associated with low redshift galaxies with narrow emission line
optical spectra, X-ray luminosities L_X(0.5-8keV)~10e41cgs and logf_X/f_opt ~
-2 suggesting `normal' star-forming galaxies. Despite the small number
statistics the sky density of `normal' X-ray selected star-forming galaxies at
the flux limit of the present sample is low consistent with previous ROSAT HRI
deep surveys. Also, the number density estimated here is in good agreement with
both the logN-logS of `normal' galaxies in the Chandra Deep Field North
(extrapolated to bright fluxes) and model predictions based on the X-ray
luminosity function of local star-forming galaxies.
