  In a previous article we proved a lower bound for the spectrum of the Dirac
operator on quaternionic Kaehler manifolds. In the present article we study the
limiting case, i. e. manifolds where the lower bound is attained as an
eigenvalue. We give an equivalent formulation in terms of a quaternionic
Killing equation and show that the only symmetric quaternionic Kaehler
manifolds with smallest possible eigenvalue are the quaternionic projective
spaces.
