Abstract: we consider the nonlinear wave equation
$$
u_{tt}-u_{xx}+\mu u + f(u)=0
$$
with Dirichlet b.c. where $\mu$ is the mass and $f(u)=u^3+O(u^5)$ the nonlinearity.

For small $\mu=1/q$, $q\in\mathbb{N}$, we find at least $O(q)$ Birkhoof-Lewis type periodic solutions with minimal period greater then $O(q)$.