Detection of magnetic fields has been reported in several sdO and sdB stars.
Recent literature has cast doubts on the reliability of most of these
detections. We revisit data previously published in the literature, and we
present new observations to clarify the question of how common magnetic fields
are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars.
About 30 of them have been observed with the FORS1 and FORS2 instruments of the
ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which
have never been observed for magnetic fields before. We also critically review
the measurements already published in the literature, and in particular we try
to explain why previous papers based on the same FORS1 data have reported
contradictory results. All new and re-reduced measurements obtained with FORS1
are shown to be consistent with non-detection of magnetic fields. We explain
previous spurious field detections from data obtained with FORS1 as due to a
non-optimal method of wavelength calibration. Field detections in other surveys
are found to be uncertain or doubtful, and certainly in need of confirmation.
There is presently no strong evidence for the occurrence of a magnetic field in
any sdB or sdO star, with typical longitudinal field uncertainties of the order
of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG
in strength occur in at most a few percent of hot subdwarfs, and may be
completely absent at this strength. Further high-precision surveys, both with
high-resolution spectropolarimeters and with instruments similar to FORS1 on
large telescopes, would be very valuable
