  Stars are not perfectly spherically symmetric. They are deformed by rotation
and magnetic fields. Until now, the study of stellar shapes has only been
possible with optical interferometry for a few of the fastest-rotating nearby
stars. We report an asteroseismic measurement, with much better precision than
interferometry, of the asphericity of an A-type star with a rotation period of
100 days. Using the fact that different modes of oscillation probe different
stellar latitudes, we infer a tiny but significant flattening of the star's
shape of $\Delta R/R = (1.8 \pm 0.6) \times 10^{-6}$. For a stellar radius $R$
that is $2.24$ times the solar radius, the difference in radius between the
equator and the poles is $\Delta R = 3 \pm 1$ km. Because the observed $\Delta
R/R$ is only one-third of the expected rotational oblateness, we conjecture the
presence of a weak magnetic field on a star that does not have an extended
convective envelope. This calls to question the origin of the magnetic field.
