Diffractive $\rho^{0}$ production at COMPASS

Diffractive leptoproduction of $\rho^{0}$ mesons, $\mu+N\rightarrow\mu+N+\rho$ is measured at COMPASS at $< W > =$ 10 GeV over a wide range of $Q^{2}$, 0.01 $< Q^{2} <$ 10 GeV$^{2}$. Angular distributions allow to determine spin density matrix elements. Preliminary results from COMPASS 2002 data are presented. They are consistent with a substantial increase of $R = \sigma_{L}/\sigma_{T}$ with Q$^{2}$ and a weak violation of SCHC, in agreement with other high energy experiments.