Feynman diagram double widggly line

I was informed that the following environment should produce a
double wiggly line

\begin{tikzpicture}
  \begin{feynman}
    (a) -- [graviton] (b),
  \end{feynman}
\end{tikzpicture}

I had impresssion that the graviton style was present in the
tikzpicture environment, but I get nothing. How do I produce a
doble wiggly line?

-- 

     Haines Brown