  Context. The origin, evolution, and ultimate fate of magnetic cataclysmic
variables are poorly understood. It is largely the nature of the magnetic
fields in these systems that leads to this poor understanding. Fundamental
properties, such as the field strength and the axis alignment, are unknown in a
majority of these systems. Aims. We undertake to put all the previous circular
polarization measurements into context and systematically survey intermediate
polars for signs of circular polarization, hence to get an indication of their
true magnetic field strengths and try to understand the evolution of magnetic
cataclysmic variables. Methods. We used the TurPol instrument at the Nordic
Optical Telescope to obtain simultaneous UBVRI photo-polarimetric observations
of a set of intermediate polars, during the epoch 2006 July 31 - August 2.
Results. Of this set of eight systems two (1RXS J213344.1+510725 and 1RXS
J173021.5-055933) were found to show significant levels of circular
polarization, varying with spin phase. Five others (V2306 Cyg, AO Psc, DQ Her,
FO Aqr, and V1223 Sgr) show some evidence for circular polarization and
variation of this with spin phase, whilst AE Aqr shows little evidence for
polarized emission. We also report the first simultaneous UBVRI photometry of
the newly identified intermediate polar 1RXS J173021.5-055933. Conclusions.
Circular polarization may be ubiquitous in intermediate polars, albeit at a low
level of one or two percent or less. It is stronger at longer wavelengths in
the visible spectrum. Our results lend further support to the possible link
between the presence of soft X-ray components and the detectability of circular
polarization in intermediate polars.
