Faint supernovae and hyper-runaway white-dwarfs from single He-detonation in double HeCO-white-dwarf mergers

We present three-dimensional hydrodynamical simulations of mergers between low-mass hybrid HeCO white dwarfs (WDs), offering new insights into the diversity of thermonuclear transients. Unlike previously studied mergers involving higher-mass HeCO WDs and CO WDs, where helium detonation often triggers core ignition, our simulations reveal incomplete helium shell detonations in comparable-mass, lower-mass WD pairs. The result is a faint, rapidly evolving transient driven by the ejection of intermediate-mass elements and radioactive isotopes such as ⁴⁸Cr and ⁵²Fe, without significant ⁵⁶Ni production. These transients may be detectable in upcoming wide-field surveys and could account for a subset of faint thermonuclear supernovae. Long-term evolution of the merger remnant shows that high-velocity PG-1159-type stars might be formed through this scenario, similar to normal CO-CO white dwarf mergers. This work expands our understanding of white dwarf mergers and their implications for nucleosynthesis and stellar evolution.