  We report the discovery and orbit of a new dwarf planet candidate, 2015
RR$_{245}$, by the Outer Solar System Origins Survey (OSSOS). 2015 RR$_{245}$'s
orbit is eccentric ($e=0.586$), with a semi-major axis near 82 au, yielding a
perihelion distance of 34 au. 2015 RR$_{245}$ has $g-r = 0.59 \pm 0.11$ and
absolute magnitude $H_{r} = 3.6 \pm 0.1$; for an assumed albedo of $p_V = 12$%
the object has a diameter of $\sim670$ km. Based on astrometric measurements
from OSSOS and Pan-STARRS1, we find that 2015 RR$_{245}$ is securely trapped on
ten-Myr timescales in the 9:2 mean-motion resonance with Neptune. It is the
first TNO identified in this resonance. On hundred-Myr timescales, particles in
2015 RR$_{245}$-like orbits depart and sometimes return to the resonance,
indicating that 2015 RR$_{245}$ likely forms part of the long-lived metastable
population of distant TNOs that drift between resonance sticking and actively
scattering via gravitational encounters with Neptune. The discovery of a 9:2
TNO stresses the role of resonances in the long-term evolution of objects in
the scattering disk, and reinforces the view that distant resonances are
heavily populated in the current Solar System. This object further motivates
detailed modelling of the transient sticking population.
