  We study the primordial scalar and tensor perturbations in inflation scenario
involving a spectator dilaton field. In our setup, the rolling spectator
dilaton causes a tachyonic instability of gauge fields, leading to a copious
production of gauge fields in the superhorizon regime, which generates
additional scalar and tensor perturbations through gravitational interactions.
Our prime concern is the possibility to enhance the tensor-to-scalar ratio $r$
relative to the standard result, while satisfying the observational
constraints. To this end, we allow the dilaton field to be stabilized before
the end of inflation, but after the CMB scales exit the horizon. We show that
for the inflaton slow roll parameter $\epsilon \gtrsim 10^{-3}$, the
tensor-to-scalar ratio in our setup can be enhanced only by a factor of ${\cal
O}(1)$ compared to the standard result. On the other hand, for smaller
$\epsilon$ corresponding to a lower inflation energy scale, a much bigger
enhancement can be achieved, so that our setup can give rise to an observably
large $r\gtrsim 10^{-2}$ even when $\epsilon\ll 10^{-3}$. The tensor
perturbation sourced by the spectator dilaton can have a strong scale
dependence, and is generically red-tilted. We also discuss a specific model to
realize our scenario, and identify the parameter region giving an observably
large $r$ for relatively low inflation energy scales.

Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology;High Energy Physics - Theory