  Given $\alpha$ in some set $\Sigma$ of total (Haar) measure in ${\bf T}={\bf
R}/{\bf Z}$, and $A\in C^{\infty}({\bf T},SL(2,{\bf R}))$ which is homotopic to
the identity, we prove that if the fibered rotation number of the skew-product
system $(\alpha,A):{\bf T}\times SL(2,{\bf R})\to {\bf T}\times SL(2,{\bf R})$,
$(\alpha,A)(\theta,y)=(\theta+\alpha,A(\theta)y)$ is diophantine with respect
to $\alpha$ and if the fibered products are uniformly bounded in the
$C^0$-topology then the cocycle $(\alpha,A)$ is $C^\infty$-reducible --that is
$A(\cdot)=B(\cdot+\alpha)A_0 B(\cdot)^{-1}$, for some $A_0\in SL(2,{\bf R})$,
$B\in C^{\infty}({\bf T},SL(2,{\bf R}))$. This result which can be seen as a
non-pertubative version of a theorem by L.H. Eliasson has two interesting
corollaries: the first one is a result of differentiable rigidity: if
$\alpha\in\Sigma$ and the cocycle $(\alpha,A)$ is $C^0$-conjugated to a
constant cocycle $(\alpha,A_0)$ with $A_0$ in a set of total measure in
$SL(2,{\bf R})$ then the conjugacy is $C^\infty$; the second consequence is: if
$\alpha\in \Sigma$ is fixed then the set of $A\in C^\infty({\bf T},SL(2,{\bf
R}))$ for which $(\alpha,A)$ has positive Lyapunov exponent is
$C^\infty$-dense. A similar result is true for the Schr\"odinger cocycle and
for 2-frequencies conservative differential equations in the plane.
