  We propose a new method of the determination of
$R^{D}=\sigma_{L}^{D}/\sigma_{T}^{D}$ from the dependence of the diffractive
cross section on the azimuthal angle between the electron scattering and proton
scattering planes. The method is based on our finding of the model independence
of the ratio of the $LT$ interference and transverse diffractive structure
functions. The predicted azimuthal asymmetry is substantial and can be measured
at HERA. We show that the accuracy of our reconstruction of $R^{D}$ is adequate
for a reliable test of an important pQCD prediction of $R^{D}\gsim 1$ for large
$\beta$.
