  We present highlights from a study of a sample of 10 extreme-luminosity
candidate ultraluminous X-ray sources (L_X > 5 x 10^(40) erg sec^(-1)), all at
distances < 100 Mpc, identified from a cross-correlation of the RC3 catalogue
of galaxies with the 2XMM catalogue. Five of the sample have also been observed
by Chandra. Of the 10 sources, seven reside in the disc or arms of spiral
galaxies, and the remaining three are close to large elliptical galaxies.
Unlike many less luminous ultraluminous X-ray sources, temporal variability is
observed on short (ks) and long (year) timescales for most sources in our
sample. Long term spectral variability is also evident in some sources. In one
case, we use archival Chandra data to demonstrate that a hyperluminous X-ray
source candidate identified by XMM-Newton is actually resolved into multiple
point sources at high spatial resolution, but note that the other candidates
remain unresolved under Chandra's intense scrutiny.
