  The cluster-tilted algebras have been introduced by Buan, Marsh and Reiten,
they are the endomorphism rings of cluster-tilting objects $T$ in cluster
categories; we call such an algebra cluster-concealed in case $T$ is obtained
from a preprojective tilting module. For example, all representation-finite
cluster-tilted algebras are cluster-concealed. If $C$ is a
representation-finite cluster-tilted algebra, then the indecomposable
$C$-modules are shown to be determined by their dimension vectors. For a
general cluster-tilted algebra $C$, we are going to describe the dimension
vectors of the indecomposable $C$-modules in terms of the root system of a
quadratic form. The roots may have both positive and negative coordinates and
we have to take absolute values.
