Can CMB data constrain the inflationary field range?

We study to what extent the spectral index ns and the tensor-to-scalar ratio r determine the field excursion Delta phi during inflation. We analyse the possible degeneracy of Delta phi by comparing three broad classes of inflationary models, with different dependence on the number of e-foldings N, to benchmark models of chaotic inflation with monomial potentials. The classes discussed cover a large set of inflationary single field models. We find that the field range is not uniquely determined for any value of (ns, r); one can have the same predictions as chaotic inflation and a very different Delta phi. Intriguingly, we find that the field range cannot exceed an upper bound that appears in different classes of models. Finally, Delta phi can even become sub-Planckian, but this requires to go beyond the single-field slow-roll paradigm