  We investigate a string-inspired dark energy scenario featuring a scalar
field with a coupling to the Gauss-Bonnet invariant. Such coupling can trigger
the onset of late dark energy domination after a scaling matter era. The
universe may then cross the phantom divide and perhaps also exit from the
acceleration. We discuss extensively the cosmological and astrophysical
implications of the coupled scalar field. Data from the Solar system,
supernovae Ia, cosmic microwave background radiation, large scale structure and
big bang nucleosynthesis is used to constrain the parameters of the model. A
good Newtonian limit may require to fix the coupling. With all the data
combined, there appears to be some tension with the nucleosynthesis bound, and
the baryon oscillation scale seems to strongly disfavor the model. These
possible problems might be overcome in more elaborate models. In addition, the
validity of these constraints in the present context is not strictly
established. Evolution of fluctuations in the scalar field and their impact to
clustering of matter is studied in detail and more model-independently. Small
scale limit is derived for the perturbations and their stability is addressed.
A divergence is found and discussed. The general equations for scalar
perturbations are also presented and solved numerically, confirming that the
Gauss-Bonnet coupling can be compatible with the observed spectrum of cosmic
microwave background radiation as well as the matter power spectrum inferred
from large scale surveys.
