  We provide a geometric characterization of manifolds of dimension 3 with
fundamental groups of which all conjugacy classes except 1 are infinite, namely
of which the von Neumann algebras are factors of type $II_1$: they are
essentially the 3-manifolds with infinite fundamental groups on which there
does not exist any Seifert fibration.
  Otherwise said and more precisely, let $M$ be a compact connected 3-manifold
and let $\Gamma$ be its fundamental group, supposed to be infinite and with at
least one finite conjugacy class besides 1. If $M$ is orientable, then $\Gamma$
is the fundamental group of a Seifert manifold; if $M$ is not orientable, then
$\Gamma$ is the fundamental group of a Seifert manifold modulo $\Bbb P$ in the
sense of Heil and Whitten \cite{HeWh--94}.
  We make heavy use of results on 3-manifolds, as well classical results (as
can be found in the books of Hempel, Jaco, and Shalen), as more recent ones
(solution of the Seifert fibred space conjecture).
