  Intermediate Polars (IPs) are a group of cataclysmic variables (CVs) which
are thought to contain white dwarfs which have a magnetic field strength in the
range ~0.1-10MG. A significant fraction of the X-ray sources detected in recent
deep surveys has been postulated to consist of IPs. Until now two of the
defining characteristics of IPs have been the presence of high (and complex)
absorption in their X-ray spectra and the presence of a stable modulation in
the X-ray light curve which is a signature of the spin period, or the beat
period, of the accreting white dwarf. Three CVs, V426 Oph, EI UMa and LS Peg,
have characteristics which are similar to IPs. However, there has been only
tentative evidence for a coherent period in their X-ray light curve. We present
the results of a search for coherent periods in XMM-Newton data of these
sources using an auto-regressive analysis which models the effects of
red-noise. We confirm the detection of a ~760 sec period in the soft X-ray
light curve of EI UMa reported by Reimer et al and agree that this represents
the spin period. We also find evidence for peaks in the power spectrum of each
source in the range 100-200 sec which are just above the 3sigma confidence
level. We do not believe that they represent genuine coherent modulations.
However, their X-ray spectra are very similar to those of known IPs. We believe
that all three CVs are bona fide IPs. We speculate that V426 Oph and LS Peg do
not show evidence for a spin period since they have closely aligned magnetic
and spin axes. We discuss the implications that this has for the defining
characteristics of IPs.
