  A new component of the cosmic medium, a light scalar field or ''quintessence
'', has been proposed recently to explain cosmic acceleration with a dynamical
cosmological constant. Such a field is expected to be coupled explicitely to
ordinary matter, unless some unknown symmetry prevents it. I investigate the
cosmological consequences of such a coupled quintessence (CQ) model, assuming
an exponential potential and a linear coupling. This model is conformally
equivalent to Brans-Dicke Lagrangians with power-law potential. I evaluate the
density perturbations on the cosmic microwave background and on the galaxy
distribution at the present and derive bounds on the coupling constant from the
comparison with observational data.
  A novel feature of CQ is that during the matter dominated era the scalar
field has a finite and almost constant energy density. This epoch, denoted as
$\phi $MDE, is responsible of several differences with respect to uncoupled
quintessence: the multipole spectrum of the microwave background is tilted at
large angles, the acoustic peaks are shifted, their amplitude is changed, and
the present 8Mpc$/h$ density variance is diminished. The present data constrain
the dimensionless coupling constant to $|\beta |\leq 0.1$.
