Is a dissipative regime during inflation in agreement with observations?

We study the spectral index of curvature perturbations for inflationary models where the driving scalar field is coupled to a relativistic fluid through a friction term Γ. In the regime in which Γ can be at most as large as H, the Hubble parameter during inflation, we find that only Γ<<H is allowed by observations; otherwise curvature fluctuations are generated with a spectral index ns unacceptably red. These results are obtained starting from vacuum initial conditions and are generic with respect to the inflationary potential and known dependence of the friction term on the scalar field and the energy density of the relativistic fluid. We compare our findings with previous investigations.