  The classification problem of Bost-Connes systems was studied by Cornellissen
and Marcolli partially, but still remains unsolved. In this paper, we will give
a representation-theoretic approach to this problem. We generalize the result
of Laca and Raeburn, which concerns with the primitive ideal space on the
Bost-Connes system for $\mathbb{Q}$. As a consequence, the Bost-Connes
$C^*$-algebra for a number field $K$ has $h_K^1$-dimensional irreducible
representations and does not have finite-dimensional irreducible
representations for the other dimensions, where $h_K^1$ is the narrow class
number of $K$. In particular, the narrow class number is an invariant of
Bost-Connes $C^*$-algebras.
