  We prove a deviation bound for the maximum of partial sums of functions of
$\alpha$-dependent sequences as defined in Dedecker, Gou{\"e}zel and
Merlev{\`e}de (2010). As a consequence, we extend the Rosenthal inequality of
Rio (2000) for $\alpha$-mixing sequences in the sense of Rosenblatt (1956) to
the larger class of $\alpha$-dependent sequences. Starting from the deviation
inequality, we obtain upper bounds for large deviations and an H{\"o}lderian
invariance principle for the Donsker line. We illustrate our results through
the example of intermittent maps of the interval, which are not $\alpha$-mixing
in the sense of Rosenblatt.
