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The widely accepted assumption is that metals detected in the spectra of afew cool helium-rich white dwarfs cannot be of primordial origin and thereforemust be accreted from the interstellar medium. However, the observed abundancesof hydrogen are much too low to be compatible with the high accretion ratesinferred from metal accretion. Hydrogen accretion is therefore suppressedcompared to metal accretion. The hypothesis most widely discussed as cause forthis ``hydrogen screening'' is the propeller mechanism: Metals are accreted inthe form of grains onto a slowly rotating, weakly magnetized white dwarf,whereas ionized hydrogen is repelled at the Alfv\'en radius. We have obtainedcircular polarization spectra of the helium-rich white dwarfs GD 40(WD0300-013) and L745-46A (WD0738-172) -- which both show strong metal lines aswell as hydrogen -- in order to search for signatures of a weak magnetic field.The magnetic field strengths necessary for the propeller mechanism to work inthese stars are at least 144000 G and 3000 G, respectively. Whereas L745-46Amight have a magnetic field of about $-$6900 G no magnetic field could be foundwith an upper limit for the field strength of 4000 G (with 99% confidence) forGD 40.Comment: 5 pages, 5 figures, Astronomy and Astrophysics, accepted for  publicatio

Do weak magnetic fields prevent hydrogen from accreting onto metal-line white dwarf stars?