An interlacing result for Hermitian matrices in Minkowski space

In this paper we will look at the well known interlacing problem, but here we consider the result for Hermitian matrices in the Minkowski space, an indefinite inner product space with one negative square. More specific, we consider the $n\times n$ matrix $A=\left[\begin{array}{cc}J& u\\ -u^{\ast }& a\end{array}\right]$ with $a\in \mathbb{R}$, $J=J^{\ast }$ and $u\in {\mathbb{C}}^{n-1}$. Then $A$ is $H$-selfadjoint with respect to the matrix $H=I_{n-1}\oplus (-1)$. The canonical form for the pair $(A,H)$ plays an important role and the sign characteristic coupled to the pair is also discussed.