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The presence of magnetic fields in O-type stars has been suspected for a longtime. The discovery of such fields would explain a wide range of welldocumented enigmatic phenomena in massive stars, in particular cyclical windvariability, H alpha emission variations, chemical peculiarity, narrow X-rayemission lines and non-thermal radio/X-ray emission.  To investigate the incidence of magnetic fields in O stars, we acquired 38new spectropolarimetric observations with FORS1 (FOcal Reducer low dispersionSpectrograph) mounted on the 8-m Kueyen telescope of the VLT.  Spectropolarimetric observations have been obtained at different phases for asample of 13 O stars. 10 stars were observed in the spectral range 348-589nm,HD36879 and HD148937 were observed in the spectral region 325-621nm, andHD155806 was observed in both settings. To prove the feasibility of the FORS1spectropolarimetric mode for the measurements of magnetic fields in hot stars,we present in addition 12 FORS1 observations of the mean longitudinal magneticfield in theta^1 Ori C and compare them with measurements obtained with theMuSiCoS, ESPaDOnS and Narval spectropolarimeters.  Most stars in our sample which have been observed on different nights show achange of the magnetic field polarity, but a field at a significance level of3sigma has been detected only in four stars, HD36879, HD148937, HD152408, andHD164794. The largest longitudinal magnetic field, <B_z> = -276+-88G, wasdetected in the Of?p star HD148937. We conclude that large-scale organisedmagnetic fields with polar field strengths larger than 1kG are not widespreadamong O-type stars.

Magnetic field measurements of O stars with FORS1 at the VLT