  We prove a reducibility result for sl(2,R) quasi-periodic cocycles close to a
constant elliptic matrix in ultra-differentiable classes, under an adapted
arithmetic condition which extends the Brjuno-R{\"u}ssmann condition in the
analytic case. The proof is based on an elementary property of the fibered
rotation number and deals with ultra-differentiable functions with a weighted
Fourier norm. We also show that a weaker arithmetic condition is necessary for
reducibility, and that it can be compared to a sufficient arithmetic condition.
