We present a method for constructing analytic expansions approximating
librational invariant curves in the case of the so-called generalized
standard map. After some preliminary changes of variables, we apply a
direct Birkhoff normal form to a fixed order. The resulting system de
scribes homotopically trivial invariant curves close to a periodic orb
it. We investigate the stability of the librational curves applying a
numerical method developed by J. Greene.