  We introduce a class of random compact metric spaces L(\alpha) indexed by
\alpha \in (1,2) and which we call stable looptrees. They are made of a
collection of random loops glued together along a tree structure, and can be
informally be viewed as dual graphs of \alpha-stable L\'evy trees. We study
their properties and prove in particular that the Hausdorff dimension of
L(\alpha) is almost surely equal to \alpha. We also show that stable looptrees
are universal scaling limits, for the Gromov-Hausdorff topology, of various
combinatorial models. In a companion paper, we prove that the stable looptree
of parameter 3/2 is the scaling limit of cluster boundaries in critical
site-percolation on large random triangulations.
