  Magnetic fields can play an important role in stellar evolution. Among white
dwarfs, the most common stellar remnant, the fraction of magnetic systems is
more than 20 per cent. The origin of magnetic fields in white dwarfs, which
show strengths ranging from 40 kG to hundreds of MG, is still a topic of
debate. In contrast, only one magnetic hot subdwarf star has been identified
out of thousands of known systems. Hot subdwarfs are formed from binary
interaction, a process often associated with the generation of magnetic fields,
and will evolve to become white dwarfs, which makes the lack of detected
magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of
three new magnetic hot subdwarfs with field strengths in the range 300-500 kG.
Like the only previously known system, they are all helium-rich O-type stars
(He-sdOs). We analysed multiple archival spectra of the three systems and
derived their stellar properties. We find that they all lack radial velocity
variability, suggesting formation via a merger channel. However, we derive
higher than typical hydrogen abundances for their spectral type, which are in
disagreement with current model predictions. Our findings suggest a lower limit
to the magnetic fraction of hot subdwarfs of 0.147 (+0.143/-0.047) per cent,
and provide evidence for merger-induced magnetic fields which could explain
white dwarfs with field strengths of 50-150 MG, assuming magnetic flux
conservation.
