  Models of planet formation and disc evolution predict a variety of
observables in the dust structure of protoplanetary discs. Here we present
Atacama Large Millimeter/submillimeter Array (ALMA) Band-6 and Band-7
observations of the transition disc Sz\,91 showing that the continuum emission
at 870$\mu$m, which is dominated by emission from large dust grains, is
localized in an optically thin narrow ring. We find that most of the emission
($\sim95\%$) is concentrated in a ring located at 110 au from the central star
that is only about 44 au wide. In contrast, the $^{12}\mathrm{CO}$ (2-1)
emission peaks closer to the star and is detected up to $\sim488$ au from the
star. The concentration of large grains in a ring-like structure while the gas
disc extends much further in and further out is in qualitative agreement with
predictions of hydrodynamical models of planet-disc interactions including
radial drift and gas drag.
