There reach’s a point on a White Dwarf where the hydrogen being siphoned from a companion star is sufficient enough that any more hydrogen landing on its surface is enough to detonate a nuclear explosion. The result is a supernova .
What if the companion star was an even smaller star than the white dwarf? There might not be enough matter to start a nuclear explosion, and so the outer surface of the larger star becomes a shell of hydrogen and helium and that’s all that’s left of the companion.......................