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 .