  Let $X=(X_t)_{t\geq 0}$ be a one-dimensional L\'evy process such that each
$X_t$ has a $C^1_b$-density w.r.t. Lebesgue measure and certain polynomial or
exponential moments. We characterize all polynomially bounded functions
$f:\mathbb{R}\to\mathbb{R}$, and exponentially bounded functions
$g:\mathbb{R}\to (0,\infty)$, such that $f(X_t)-\mathbb{E} f(X_t)$, resp.
$g(X_t)/\mathbb{E} g(X_t)$, are martingales.
