The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, H$\alpha$ emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. We present the results of our studies of magnetic fields in O-type stars, carried out over the last years, and give an overview about the fundamental parameters of the stars in which a magnetic field was detected.
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