The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000K. This is far beyond what would be expected for a normal core helium burning star. Theories to explain these stars include binary interactions, and "late thermal pulses", where a thermal pulse that Asymptotic giant branch (AGB) stars experience regularly, occurs after fusion has ceased and the star has entered the superwind phase. These stars are "born again" with unusual properties. Despite the bizarre-sounding process, this is expected to occur for 10% or more of post-AGB stars, although it is thought that only particularly late thermal pulses create extreme horizontal-branch stars, after the planetary nebular phase and when the central star is already cooling towards a white dwarf.