  I examine the stability of retrograde mixed modes in rotating B-type stars.
These modes can be regarded as a hybrid between the Rossby modes that arise
from conservation of vorticity, and the Poincare modes that are gravity waves
modified by the Coriolis force. Using a non-adiabatic pulsation code based
around the traditional approximation, I find that the modes are unstable in
mid- to late-B type stars, due to the same iron-bump opacity mechanism usually
associated with SPB and $\beta$ Cep stars. At one half of the critical rotation
rate, the instability for $m=1...4$ modes spans the spectral types B4 to A0.
Inertial-frame periods of the unstable modes range from 100 days down to a
fraction of a day, while normalized growth rates can reach in excess of
$10^{-5}$.
  I discuss the relevance of these findings to SPB and pulsating Be stars, and
to the putative Maia class of variable star. I also outline some of the
questions raised by this discovery of a wholly-new class of pulsational
instability in early-type stars.
