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We present highly time resolved circular-polarization and flux spectra of themagnetic white dwarf LP790-29 taken with the VLT UT1 in order to test thehypothesis that LP790-29 is a fast rotator with a period of the order ofseconds to minutes. Due to low time resolution of former observations thismight have been overlooked -- leading to the conclusion that LP790-29 has arotational period of over 100 years. The optical spectrum exhibits oneprominent absorption feature with minima at about 4500, 4950, and 5350 A, whichare most likely C2 Swan-bands shifted by about 180 A in a magnetic fieldbetween 50MG and 200MG. At the position of the absorption structures the degreeof circular polarization varies between -1% and +1%, whereas it amounts to +8to +10% in the blue and red continuum. With this very high degree ofpolarization lp790-29 is very well suited to a search for short timevariations, since a variation of several percent in the polarization can beexpected for a magnetic field oblique to the rotational axis. From our analysiswe conclude that variations on time scales from 50 to 2500 seconds must haveamplitudes <0.7% in the continuum and <2% in the strongest absorption featureat 4950A. While no short-term variations could be found a careful comparison ofour polarization data of LP790-29 with those in the literatures indicatessignificant variations on time scales of decades with a possible period ofabout 24-28 years.Comment: 5 pages, 4 figures. Astronomy and Astrophysics, in pres

Search for variations in circular-polarization spectra of the magnetic white dwarf LP 790–29