  Properties of low-dimensional spin-Peierls systems are described by using a
one dimensional S=1/2 antiferromagnetic Heisenberg chain linearly coupled to a
single phonon mode of wave vector pi (whose contribution is expected to be
dominant). By exact diagonalizations of small rings with up to 24 sites
supplemented by a finite size scaling analysis, static and dynamical properties
are investigated. Numerical evidences are given for a spontaneous discrete
symmetry breaking towards a spin gapped phase with a frozen lattice
dimerization. Special emphasis is put on the comparative study of the two
inorganic spin-Peierls compounds CuGeO3 and NaV2O5 and the model parameters are
determined from a fit of the experimental spin gaps. We predict that the
spin-phonon coupling is 2 or 3 times larger in NaV2O5 than in CuGeO3. Inelastic
neutron scattering spectra are calculated and similar results are found in the
single phonon mode approximation and in the model including a static
dimerization. In particular, the magnon S=1 branch is clearly separated from
the continuum of triplet excitations by a finite gap.
