  We study the influence of Szeg{\"o} projector $\Pi$ on the L 2 --critical
one-dimensional non linear focusing Schr{\"o}dinger equation, leading to the
quintic focusing NLS-Szeg{\"o} equation i$\partial$ t u + $\partial$ 2 x u +
$\Pi$(|u| 4 u) = 0, (t, x) $\in$ R x R, u(0, $\times$) = u 0. This equation is
globally well-posed in H 1 + = $\Pi$(H 1 (R)), for every initial datum u 0. The
solution L 2-scatters both forward and backward in time if u 0 has sufficiently
small mass. We prove the orbital stability with scaling of the traveling wave :
u $\omega$,c (t, x) = e i$\omega$t Q(x + ct), for some $\omega$, c > 0, where Q
is a ground state associated to Gagliardo-Nirenberg type functional I
($\gamma$) (f) = $\partial$ x f 2 L 2 f 4 L 2 + $\gamma$ --i$\partial$ x f, f 2
L 2 f 2 L 2 f 6 L 6 , $\forall$f $\in$ H 1 + \{0}, for some $\gamma$ $\ge$ 0.
The ground states are completely classified in the case $\gamma$ = 2, leading
to the actual orbital stability without scaling for appropriate traveling
waves. As a consequence, the scattering mass threshold of the focusing quintic
NLS-Szeg{\"o} equation is strictly below the mass of ground state associated to
the functional I (0) , unlike the recent result by Dodson [6] on the usual
quintic focusing non linear Schr{\"o}dinger equation.
