  Let $G_n=U(n)\ltimes {\mathbb H}_n $ be the semi-direct product of the
unitary group acting by automorphisms on the Heisenberg group ${\mathbb H}_n$.
According to Lipsman, the unitary dual $\widehat {G_n} $ of $G_n $ is in one to
one correspondence with the space of admissible coadjoint orbits $\mathfrak
g_n^\ddagger /G_n $ of $G_n $. In this paper, we determine the topology of the
space $\mathfrak g_n^\ddagger /G_n $ and we show that the correspondence with
$\widehat {G_n} $ is a homeomorphism.
