  The Arnoux-Rauzy systems are defined in \cite{ar}, both as symbolic systems
on three letters and exchanges of six intervals on the circle. In connection
with a conjecture of S.P. Novikov, we investigate the dynamical properties of
the interval exchanges, and precise their relation with the symbolic systems,
which was known only to be a semi-conjugacy; in order to do this, we define a
new system which is an exchange of nine intervals on the line (it was described
in \cite{abb} for a particular case). Our main result is that the
semi-conjugacy determines a measure-theoretic isomorphism (between the three
systems) under a diophantine (sufficient) condition, which is satisfied by
almost all Arnoux-Rauzy systems for a suitable measure; but, under another
condition, the interval exchanges are not uniquely ergodic and the isomorphism
does not hold for all invariant measures; finally, we give conditions for these
interval exchanges to be weakly mixing.
