High velocity stars are messengers of violent astrophysical phenomena.  Either unleashed from the Galactic centre (hypervelocity stars), or ejected  from stellar clusters and binary supernova explosions (runaway stars),  these stars are powerful tools to study distant stellar populations and  rare physics. Thermonuclear explosions, like those  occurring in Type Ia supernovae, are expected to entirely disrupt an  accreting white dwarf in a compact binary system, leading to the  ejection of their companion stars at high velocities.
Recent observations, also guided by the astrometric Gaia mission, have  revealed new exotic objects that may form via the (failed) thermonuclear  explosion of white dwarfs. Among the new discoveries we find unusual  white dwarfs with carbon or oxygen-dominated atmospheres,  as well as stars with neon-dominated atmospheres that could be the formerly  accreting white dwarf survivors of peculiar thermonuclear supernovae.
As we are just starting to discover these rare objects, the accurate  characterisation of their unusual atmospheres is improving our understanding  of the formation scenarios and evolution, having an impact theoretical  works that aim to constrain the pathways to thermonuclear supernovae,  birth rates, and nucleosynthetic yields.