  We study the existence of centers of planar autonomous system of the form
  $$(S) \quad \dot x=y,\qquad \dot y = -h(x) - g(x)y - f(x)y^2.$$
  We are interested in the period function $T$ around a center 0. A sufficient
condition for the isochronicity of (S) at 0 is given. Such a condition is also
necessary when $f,g,h$ are analytic functions. In that case a characterization
of isochronous centers of system (S) is given. Some applications will be
derived. In particular, new families of isochronous centers will be described
