  The widely accepted assumption is that metals detected in the spectra of a
few cool helium-rich white dwarfs cannot be of primordial origin and therefore
must be accreted from the interstellar medium. However, the observed abundances
of hydrogen are much too low to be compatible with the high accretion rates
inferred from metal accretion. Hydrogen accretion is therefore suppressed
compared to metal accretion. The hypothesis most widely discussed as cause for
this ``hydrogen screening'' is the propeller mechanism: Metals are accreted in
the form of grains onto a slowly rotating, weakly magnetized white dwarf,
whereas ionized hydrogen is repelled at the Alfv\'en radius. We have obtained
circular polarization spectra of the helium-rich white dwarfs GD 40
(WD0300-013) and L745-46A (WD0738-172) -- which both show strong metal lines as
well as hydrogen -- in order to search for signatures of a weak magnetic field.
The magnetic field strengths necessary for the propeller mechanism to work in
these stars are at least 144000 G and 3000 G, respectively. Whereas L745-46A
might have a magnetic field of about $-$6900 G no magnetic field could be found
with an upper limit for the field strength of 4000 G (with 99% confidence) for
GD 40.
