  We consider the excitation of radial and non-radial oscillations in low-mass
B stars by the iron-bump opacity mechanism. The results are significant for the
interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and
extreme helium stars, including the EC14026 and PG1716 variables. We
demonstrate that, for radial oscillations, the driving mechanism becomes
effective by increasing the contrast between the iron-bump opacity and the
opacity from other sources. The location of the iron-bump instability boundary
depends on the mean molecular weight in the envelope and also on the radial
order of the oscillation. A bluer instability boundary is provided by
increasing the iron abundance alone, explaining the observed EC14026 variables,
and by higher radial order oscillations. We show that the coolest EC14026
variables may vary in the fundamental radial mode, but the hottest variables
must be of higher radial order. In considering non-radial oscillations, we
demonstrate that g-modes of high radial order and low spherical degree (l<4)
may be excited in some blue horizontal branch stars with near-normal
composition (Z=0.02). Additional iron enhancement extends the g-mode
instability zone to higher effective temperatures and also creates a p-mode
instability zone. With sufficient iron, the p-mode and g-mode instability zones
overlap, allowing a small region where the EC14026 and PG1716-type variability
can be excited simultaneously. However its location is roughly 5000 K too low
compared with the observed boundary between EC14026 and PG1716 variables.
