  We prove the spectral instability of the complex cubic oscillator
$-\frac{d^2}{dx^2}+ix^3+i\alpha x$ for non-negative values of the parameter
$\alpha$, by getting the exponential growth rate of $\|\Pi_n(\alpha)\|$, where
$\Pi_n(\alpha)$ is the spectral projection associated with the $n$-th
eigenvalue of the operator. More precisely, we show that for all non-negative
$\alpha$ \[
  \lim\limits_{n\to+\infty}\frac{1}{n}\log\|\Pi_n(\alpha)\| =
\frac{\pi}{\sqrt{3}}. \]
