  CP Tuc (AX J2315--592) shows a dip in X-rays which lasts for approximately
half the binary orbit and is deeper in soft X-rays compared with hard X-rays.
It has been proposed that this dip is due to the accretion stream obscuring the
accretion region from view. If CP Tuc was a polar, as has been suggested, then
the length of such a dip would make it unique amongst polars since in those
polars in which a dip is seen in hard X-rays the dip lasts for only 0.1 of the
orbit. We present optical polarimetry and RXTE observations of CP Tuc which
show circular polarisation levels of ~10 per cent and find evidence for only
one photometric period. These data confirm CP Tuc as a polar. Our modelling of
the polarisation data imply that the X-ray dip is due to the bulk of the
primary accretion region being self-eclipsed by the white dwarf. The energy
dependence of the dip is due to a combination of this self-eclipse and also the
presence of an X-ray temperature gradient over the primary accretion region.
