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.Comment: 43 pages. In version 2, at the referee's suggestion, we slightly
expand some statements (Theorem 1.2 and Proposition 3.19) to include the
relevant affine varieties. This version is to appear in Advances in
Mathematic