  Spectropolarimetic campaigns have established that large-scale magnetic
fields are present at the surfaces of approximately 10% of massive dwarf stars.
However, there is a dearth of magnetic field measurements for their deep
interiors. Asteroseismology of gravity-mode pulsations combined with rotating
magneto-hydrodynamical calculations of the early-B main-sequence star HD 43317
constrain its magnetic field strength to be approximately $5\times10^{5}$ G
just outside its convective core. This proof-of-concept study for
magneto-asteroseismology opens a new window into the observational
characterisation of magnetic fields inside massive stars.
