 Low-mass helium white dwarfs (with a mass < 0.20 M ☉, often referred to as "extremely low-mass white dwarfs, ELM WDs") are formed in binary systems. As a result of their hydrogen-rich envelopes, residual hydrogen burning via the CNO cycle may keep these white dwarfs hot on a long timescale. In addition, they remain in a bloated proto-white dwarf stage for up to 2 Gyr before they reach the cooling track.
