  The Cartwheel is one of the most outstanding examples of a dynamically
perturbed galaxy where star formation is occurring inside the ring--like
structure. In previous studies with Chandra, we detected 16 Ultra Luminous
X-ray sources lying along the southern portion of the ring. Their Luminosity
Function is consistent with them being in the high luminosity tail of the High
Mass X-ray Binaries distribution, but with one exception: source N.10. This
source, detected with Chandra at L_X = 1.x 10^(41) ergs, is among the brightest
non--nuclear sources ever seen in external galaxies. Recently, we have observed
the Cartwheel with XMM-Newton in two epochs, six months apart. After having
been at its brightest for at least 4 years, the source has dimmed by at least a
factor of two between the two observations. This fact implies that the source
is compact in nature. Given its extreme isotropic luminosity, there is the
possibility that the source hosts an accreting intermediate--mass black hole.
Other sources in the ring vary in flux between the different datasets. We
discuss our findings in the context of ULX models.
