  We show that the fixed-point subvariety of a Nakajima quiver variety under a
diagram automorphism is a disconnected union of quiver varieties for the
`split-quotient quiver' introduced by Reiten and Riedtmann. As a special case,
quiver varieties of type D arise as the connected components of fixed-point
subvarieties of diagram involutions of quiver varieties of type A. In the case
where the quiver varieties of type A correspond to small self-dual
representations, we show that the diagram involutions coincide with classical
involutions of two-row Slodowy varieties. It follows that certain quiver
varieties of type D are isomorphic to Slodowy varieties for orthogonal or
symplectic Lie algebras.
