Following the discovery of blue large-amplitude pulsators (BLAPs) by the OGLE survey, additional hot, high-amplitude pulsating stars have been discovered by the Zwicky Transient Facility. It has been proposed that all of these objects are low-mass pre-white dwarfs and that their pulsations are driven by the opacity of iron-group elements. With this expanded population of pulsating objects, it was decided to compute a sequence of post-common-envelope stellar models using the mesa stellar evolution code and to examine the pulsation properties of low-mass pre-white dwarfs using non-adiabatic analysis with the gyre stellar oscillation code. By including the effects of atomic diffusion and radiative levitation, it is shown that a large region of instability exists from effective temperatures of 30 000 K up to temperatures of at least 50 000 K and at a wide range of surface gravities. This encompasses both groups of pulsator observed so far, and confirms that the driving mechanism is through iron group element opacity. We make some conservative estimates about the range of periods, masses, temperatures and gravities in which further such pulsators might be observed.

Following the recent discovery of a new class of pulsating star, the blue large-amplitude pulsators (BLAPs), pulsation stability analysis was carried out on evolutionary models of post-common envelope (CE) ejection stars of 0.3 and 0.46 solar masses. These models subsequently evolve to become a low-mass helium white dwarf and a core helium-burning extreme horizontal-branch star respectively. We investigate the effects of atomic diffusion, particularly radiative levitation, on the pulsation behaviour of the models. We find that when the models have effective temperatures comparable to those of BLAPs, the inclusion of radiative levitation allows sufficient enhancement of heavy metals to produce opacity-driven fundamental mode pulsations with periods similar to those observed in BLAPs.

Diffusion of elements in the atmosphere and envelope of a star can drastically alter its surface composition, leading to extreme chemical peculiarities. We consider the case of hot subdwarfs, where surface helium abundances range from practically zero to almost 100 percent. Since hot subdwarfs can form via a number of different evolution channels, a key question concerns how the formation mechanism is connected to the present surface chemistry. A sequence of extreme horizontal branch star models was generated by producing post-common envelope stars from red giants. Evolution was computed with MESA from envelope ejection up to core-helium ignition. Surface abundances were calculated at the zero-age horizontal branch for models with and without diffusion. A number of simulations also included radiative levitation. The goal was to study surface chemistry during evolution from cool giant to hot subdwarf and determine when the characteristic subdwarf surface is established. Only stars leaving the giant branch close to core-helium ignition become hydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, including radiative levitation, depletes the initial surface helium in all cases. All subdwarf models rapidly become more depleted than observations allow. Surface abundances of other elements follow observed trends in general, but not in detail. Additional physics is required.