  We present highly time resolved circular-polarization and flux spectra of the
magnetic white dwarf LP790-29 taken with the VLT UT1 in order to test the
hypothesis that LP790-29 is a fast rotator with a period of the order of
seconds to minutes. Due to low time resolution of former observations this
might have been overlooked -- leading to the conclusion that LP790-29 has a
rotational period of over 100 years. The optical spectrum exhibits one
prominent absorption feature with minima at about 4500, 4950, and 5350 A, which
are most likely C2 Swan-bands shifted by about 180 A in a magnetic field
between 50MG and 200MG. At the position of the absorption structures the degree
of circular polarization varies between -1% and +1%, whereas it amounts to +8
to +10% in the blue and red continuum. With this very high degree of
polarization lp790-29 is very well suited to a search for short time
variations, since a variation of several percent in the polarization can be
expected for a magnetic field oblique to the rotational axis. From our analysis
we conclude that variations on time scales from 50 to 2500 seconds must have
amplitudes <0.7% in the continuum and <2% in the strongest absorption feature
at 4950A. While no short-term variations could be found a careful comparison of
our polarization data of LP790-29 with those in the literatures indicates
significant variations on time scales of decades with a possible period of
about 24-28 years.
