  In this paper I argue that, far from necessarily hindering bar formation in
disc galaxies, inner haloes may stimulate it. This constitutes a new
instability mechanism by which bars can grow. To show this I use a number of
N-body simulations whose initial conditions have identical discs and more or
less concentrated haloes. They show that the bar that grows in the more
halo-dominated environment is considerably stronger than the bar that grows in
the more disc-dominated environment. This result is obtained from simulations
with live haloes, i.e. composed of particles which respond to the disc and take
part in the evolution. On the other hand, if the halo is rigid, it hinders or
quenches bar formation, as expected. Comparison of two simulations which are
identical in everything, except that the halo is live in the first one and
rigid in the second one, leads me to suggest that the halo response can help
the bar grow. Following the orbits of the stars in the halo, I find that a
considerable fraction of the halo particles are in resonance with the bar. The
halo may thus take angular momentum from the bar and stimulate its growth. I
finally discuss whether and how the results of the N-body simulations can be
applied to real galaxies.
