  We explore the X-ray properties of a subset of the optically selected SDSS
cluster sample of Goto et al. (2002), by analysing seven public XMM-{\em
Newton} pointings, with exposure times ranging from $\sim$ 4 to 46 ksec. There
are in total 17 SDSS clusters out of which only eight are detected at X-ray
wavelengths with $\rm f_{0.5-2keV}\magcir 1.2 \times 10^{-14}$ ergs cm$^{-2}$
s$^{-1}$. For the remaining 9 SDSS clusters we estimate their 3$\sigma$
luminosity upper limits (corresponding to $L_x\mincir 5\times 10^{42}$ ergs/sec
in the 0.5-2 keV band). This relatively low luminosity suggests that if real
structures, these galaxy aggregations correspond to poor groups of galaxies.
Using the SDSS photometric catalogue we also derive the cluster optical
$r$-band luminosities. The resulting scaling relations ($L_{\rm opt}-L_x$,
$L_{\rm opt}-T_x$) are consistent with those of other recent studies.

Astrophysics