  The radial-orbit instability is a collective phenomenon that has heretofore
only been observed in spherical systems. We find that this instability occurs
also in triaxial systems, as we checked by performing extensive N-body
simulations whose initial conditions were obtained by sampling a
self-consistent triaxial model of a cuspy galaxy composed of luminous and dark
matter. N-body simulations show a time evolution of the galaxy that is not due
to the development of chaotic motions but, rather, to the collective
instability induced by an excess of box-like orbits. The instability quickly
transforms such models into a more prolate configuration, with 0.64<b/a<0.77
and 0.6<c/a<0.7 for the dark halo and 0.64<b/a<0.77 and 0.59<c/a<0.67 for the
luminous matter. Stable triaxial, cuspy galaxies with dark matter halos are
obtained when the contribution of radially-biased orbits to the solution is
reduced. These results constitute the first evidence of the radial-orbit
instability in triaxial galaxy models.
