Formation and evolution of hybrid He-CO white dwarfs and their properties

White dwarfs (WDs) are the stellar core remnants of low mass (≲ 8 M_{⊙}) stars. They are typically divided into three main composition groups: oxygen-neon (ONe), carbon-oxygen (CO), and helium (He) WDs. The evolution of binary systems can significantly change the evolution of the binary stellar components. In particular, striping the envelope of an evolved star can give rise to a core remnant, which can later evolve into a WD with significantly different composition. Here we focus on the formation and evolution of hybrid HeCO WDs. We follow the formation and stellar evolution of such WDs for a range of initial conditions and provide their detailed structure, mass-radius relation and luminosity-temperature evolution. We confirm that both low-mass WDs (< 0.45 M_{⊙}, typically thought to be He-WDs) and intermediate-mass WDs (0.45 < M_WD ≤ 0.7, typically thought to be CO-WDs) could in fact be hybrid HeCO WDs, with 5-25 (75-95) per cent of their mass in He (CO). We use population synthesis calculations to infer the birth rate and properties of such WDs. We find that hybrid HeCO-WD comprise the majority of young (<2 Gyr) WDs in binaries, but are rarer among older WDs in binaries. The high frequency and large He content of such WDs could have an important role in WD-WD mergers, and may give rise to sub-Chandrasekhar thermonuclear supernova explosions.