  We report results from a systematic strong-coupling expansion of a spin-1/2
Heisenberg chain coupled to Einstein phonons. In the non-adiabatic regime this
model is used to describe zero temperature properties of CuGeO3. The linked
cluster expansion allows the determination of observables in the thermodynamic
limit preserving the full lattice dynamics without a truncation of the phononic
Hilbert space. In particular, the spin gap and the dispersion of the elementary
triplet excitation are calculated up to 10th order in a dimer expansion. The
magnetic structure factor of the ground state is evaluated up to 6th order. We
show that the spin-phonon coupling leads to a renormalization of the elementary
triplet dispersion. Surprisingly in the non-adiabatic regime a substantial
renormalization of the spin gap only sets in at much larger couplings than
those proposed for CuGeO3. The ground state magnetic correlations are found to
be hardly effected by the spin-phonon coupling, but dominated by the
frustrating magnetic interaction in the parameter regime relevant for CuGeO3.
