Discovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors

We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M $\leqslant$ 0.3 M$_{\odot}$) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320--590 s, consistent in timescale with theoretical predictions of $p$-mode pulsations in mixed-atmosphere $\approx$ 0.18 M$_{\odot}$ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, time-series photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs.