Polarization of IPs

Strongly polarized optical/IR emission is a defining characteristic of polars; the studies of cyclotron humps and Zeeman-split photospheric features have allowed direct measurements of the magnetic field strength in polars.

This is not the case in IPs - there are no reports of cyclotron humps or Zeeman-split features. Polarization has been detected in 6 or so systems, however, allowing some constraints to be set on the magnetic field strengths. Unfortunately, the analysis is more complicated because any cyclotron emission can be diluted by unpolarized emissions from the partial accretion disk, the bright spot, and the irradiated disk/bright spot/secondary.

Wickramasinghe et al. 1991 suggest non-detection of polarization suggests B < 5 MG. If this is confirmed, then there are quite a few IPs with magnetic fields weaker than known polars, and hence will not evolve into polars. There are a few IPs that will.

Survey and Non-system-specific Modelling

Cropper 1986: No variable polarization is found in FO Aqr, AE Aqr, EX Hya, AO Psc, or V1223 Sgr; low level non-variable linear polarization appears to be present in some.
Butters et al. 2009: In addition to 1RXS J213344.1+510725 reported earlier, V2731 Oph is found to show significant, spin-modulated circular polarization. shows little evidence of polarization, while FO Aqr, V2306 Cyg, DQ Her, AO Psc, and V1223 Sgr show some evidence of polarization.
Stockman et al. 1992: 4-sigma result on AE Aqr, 3-sigma result on FO Aqr, and non-detections of TV Col, DO Dra, DQ Her, EX Hya, GK Per, and AO Psc.
Wickramasinghe et al. 1991: non-detection of polarized light suggests B < 5 MG