2007A&A...462.1097J

Weak magnetic fields have been searched for on only a small number of white dwarfs. Current estimates find that about 10% of all white dwarfs have fields in excess of 1MG; according to previous studies this number increases up to about 25% in the kG regime. Our aim is to improve on these statistics by a new sample of ten white dwarfs in order to determine the ratio of magnetic to field-free white dwarfs. Mean longitudinal magnetic fields strengths were determined by means of high-precision circular polarimetry of Hβ and Hγ with the FORS1 spectrograph of the VLT ``Kueyen'' 8m telescope. In one of our objects (LTT7987), we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between (-1±0.5)kG and (+1±0.5)kG. This would be the weakest magnetic field ever found in a white dwarf, but systematic errors cannot completely be ruled out at this level of accuracy. We also observed the sdO star EC11481-2303 but could not detect a magnetic field. VLT observations with uncertainties typically of 1000G or less suggest that 15-20% of WDs have kG fields. Together with previous investigations, the fraction of kG magnetic fields in white dwarfs amounts to about 11-15%, which is close to the current estimations for highly magnetic white dwarfs (>1MG).