Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly sinusoidal variation over the WD spin cycle, and almost symmetric positive and negative polarization excursions. Maximum amplitudes are observed in the B and V bands (±3%). This is the first time that polarization peaking in the blue has been discovered in an intermediate polar (IP), and suggests that V405 Aur is the highest magnetic field IP found so far. The polarized flux spectrum is similar to those found in polars with magnetic fields in the range B ~ 25-50 MG. Our low-resolution circular spectropolarimetry has given evidence of transient features which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B = 31.5 ± 0.8 MG, consistent with the broadband polarized flux spectrum. Timings of the circular polarization zero crossovers put strict upper limits on WD spin period changes, and indicate that the WD in V405 Aur is currently accreting closely at the spin equilibrium rate, with very long synchronization timescales, Ts 109 yr. For the observed spin to orbital period ratio, Pspin/Porb = 0.0365, and Porb ~ 4.15 hr, existing numerical accretion models predict spin equilibrium condition with B ~ 30 MG if the mass ratio of the binary components is q1 ~ 0.4. The high magnetic field makes V405 Aur a likely candidate as a progenitor of a polar.
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