  We investigate the following question: what is the set of unit volume which
can be best irrigated starting from a single source at the origin, in the sense
of branched transport? We may formulate this question as a shape optimization
problem and prove existence of solutions, which can be considered as a sort of
"unit ball" for branched transport. We establish some elementary properties of
optimizers and describe these optimal sets A as sublevel sets of a so-called
landscape function which is now classical in branched transport. We prove
$\beta$-H{\"o}lder regularity of the landscape function, allowing us to get an
upper bound on the Minkowski dimension of the boundary: dim $\partial$A $\le$ d
-- $\beta$ (where $\beta$ := d($\alpha$ -- (1 -- 1/d)) $\in$ (0, 1) is a
relevant exponent in branched transport, associated with the exponent $\alpha$
> 1 -- 1/d appearing in the cost). We are not able to prove the upper bound,
but we conjecture that $\partial$A is of non-integer dimension d -- $\beta$.
Finally, we make an attempt to compute numerically an optimal shape, using an
adaptation of the phase-field approximation of branched transport introduced
some years ago by Oudet and the second author.
