Wikipedia
Cyclocarbon
A cyclo[n]carbon is a chemical compound consisting solely of an n number of carbon atoms linked in a ring by double covalent bonds (a cyclic cumulene). Since the compound is composed only of carbon atoms, it is considered an exotic carbon allotropes. If n is even there is an alternative structure where the bonds alternate between single and triple bonds (a cyclic polyyne). The (hypothetical) six-carbon member of this family (C) is also called benzotriyne.
The smallest cyclo[n]carbon predicted to be thermodynamically stable is C, with a computed strain energy of 72 kilocalories per mole.