  The background due to the direct diffractive dissociation of the photon into
the $\pi^+\pi^-$-pair to the "elastic" diffractive $\rho^0$-meson production in
electron-proton collisions is calculated. At large $Q^2$ the interference
between resonant and non-resonant $\pi^+ \pi^-$ production changes the
$\sigma^L/\sigma^T$ ratio with the mass of the $2\pi$ (i.e. $\rho^0$-meson)
state.
