  Let $\mathbf{A}$ be a bounded self-adjoint operator on a separable Hilbert
space $\mathfrak{H}$ and $\mathfrak{H}_0\subset\mathfrak{H}$ a closed invariant
subspace of $\mathbf{A}$. Assuming that $\mathfrak{H}_0$ is of codimension 1,
we study the variation of the invariant subspace $\mathfrak{H}_0$ under bounded
self-adjoint perturbations $\mathbf{V}$ of $\mathbf{A}$ that are off-diagonal
with respect to the decomposition $\mathfrak{H}=
\mathfrak{H}_0\oplus\mathfrak{H}_1$. In particular, we prove the existence of a
one-parameter family of dense non-closed invariant subspaces of the operator
$\mathbf{A}+\mathbf{V}$ provided that this operator has a nonempty singularly
continuous spectrum. We show that such subspaces are related to non-closable
densely defined solutions of the operator Riccati equation associated with
generalized eigenfunctions corresponding to the singularly continuous spectrum
of $\mathbf{B}$.
