  We present ALMA (Cycle 0) band-6 and band-3 observations of the transition
disk Sz\,91. The disk inclination and position angle are determined to be
$i=49.5\degr\pm3.5\degr$ and $\mathrm{PA}=18.2\degr\pm3.5\degr$ and the dusty
and gaseous disk are detected up to $\sim220$ au and $\sim400$ au from the
star, respectively. Most importantly, our continuum observations indicate that
the cavity size in the mm-sized dust distribution must be $\sim97$ au in
radius, the largest cavity observed around a T Tauri star. Our data clearly
confirms the presence of \co(2-1) well inside the dust cavity. Based on these
observational constrains we developed a disk model that simultaneously accounts
for the \co and continuum observations (i.e., gaseous and dusty disk).
According to our model, most of the millimeter emission comes from a ring
located between 97 and 140 au. We also find that the dust cavity is divided
into an innermost region largely depleted of dust particles ranging from the
dust sublimation radius up to 85 au, and a second, moderately dust-depleted
region, extending from 85 to 97 au. The extremely large size of the dust
cavity, the presence of gas and small dust particles within the cavity and the
accretion rate of Sz\,91 are consistent with the formation of multiple (giant)
planets.
