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 .
This can be achieved either by increasing the iron abundance or by decreasing the hydrogen abundance .
The location of the iron-bump instability boundary is found to depend 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 , rather than the entire metal component , and is required to explain the observed EC14026 variables .
A bluer instability boundary is also provided by higher radial order oscillations .
Using data for observed and theoretical period ranges , we show that the coolest EC14026 variables may vary in the fundamental radial mode , but the hottest variables must vary in modes 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 .
The latter is essentially contiguous with the radial 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 .
The overlap zone is principally a function of effective temperature and only weakly a function of luminosity .
However its location is roughly 5000K too low compared with the observed boundary between EC14026 and PG1716 variables .
The discrepancy can not be resolved by simply increasing the iron abundance .