  Among the four known transneptunian dwarf planets, Haumea is an exotic, very
elongated, and fast rotating body. In contrast to the other dwarf planets, its
size, shape, albedo, and density are not well constrained. Here we report
results of a multi-chord stellar occultation, observed on 2017 January 21.
Secondary events observed around the main body are consistent with the presence
of a ring of opacity 0.5, width 70 km, and radius 2,287$_{-45}^{+75}$ km. The
Centaur Chariklo was the first body other than a giant planet to show a ring
system and the Centaur Chiron was later found to possess something similar to
Chariklo's rings. Haumea is the first body outside the Centaur population with
a ring. The ring is coplanar with both Haumea's equator and the orbit of its
satellite Hi'iaka. Its radius places close to the 3:1 mean motion resonance
with Haumea's spin period. The occultation by the main body provides an
instantaneous elliptical limb with axes 1,704 $\pm$ 4 km x 1,138 $\pm$ 26 km.
Combined with rotational light-curves, it constrains Haumea's 3D orientation
and its triaxial shape, which is inconsistent with a homogeneous body in
hydrostatic equilibrium. Haumea's largest axis is at least 2,322 $\pm$ 60 km,
larger than thought before. This implies an upper limit of 1,885 $\pm$ 80 kg
m$^{-3}$ for Haumea's density, smaller and less puzzling than previous
estimations, and a geometric albedo of 0.51 $\pm$ 0.02, also smaller than
previous estimations. No global N$_2$ or CH$_4$ atmosphere with pressures
larger than 15 and 50 nbar (3-$\sigma$ limits), respectively, is detected.
