Inflationary predictions in scalar-tensor DBI inflation
  The scalar-tensor Dirac-Born-Infeld (DBI) inflation scenario provides a
simple mechanism to reduce the large values of the boost factor associated with
single field models with DBI action, whilst still being able to drive 60 efolds
of inflation. Using a slow-roll approach, we obtain an analytical expression
for the spectral index of the perturbations and, moreover, determine
numerically the regions of the parameter space of the model capable of giving
rise to a power spectrum with amplitude and spectral index within the observed
bounds. We find that regions that exhibit significant DBI effects throughout
the inflationary period can be discarded by virtue of a blue-tilted spectral
index, however, there are a number of viable cases --- associated with a more
red-tilted spectral index --- for which the boost factor is initially
suppressed by the effect of the coupling between the fields, but increases
later to moderate values.
