  We report the identification of the polarization of ground state absorption
lines from post-AGB 89~Hercules. Two ground state neutral iron lines are found
to have counterintuitive high-amplitude polarizations and an unchanged
polarization direction through the orbital period, as opposed to the pattern of
polarizations of absorption lines from excited states, which are synchronized
with the orbital phase owing to optical pumping. This can be explained with
magnetic realignment on the ground state. The 3D mean magnetic field is thereby
unveiled from the degree and direction of the polarizations of the two iron
lines. The field strength is also constrained to be $\lesssim 100\,{\rm mG}$.
Our result has thus improved the accuracy by orders of magnitude compared to
the previous $10\,{\rm G}$ upper limit set by non-detection of the Zeeman
effect.
