  We consider perturbations of the one-dimensional cubic Schr\"odinger
equation, of the form $i \, \partial_t \psi + \partial_x^2 \psi + |\psi|^2 \psi
+ g( |\psi|^2 ) \psi = 0$. Under hypotheses on the function $g$ that can be
easily verified in some cases (such as $g(s) = s^\sigma$ with $\sigma >1$), we
show that the linearized problem around a small solitary wave presents a unique
internal mode. Moreover, under an additional hypothesis (the Fermi golden rule)
that can also be verified in the case of powers $g(s) = s^\sigma$, we prove the
asymptotic stability of the solitary waves with small frequencies.
