Search for variations in circular-polarization spectra of the magnetic white dwarf LP790-29

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.