“…Conjecturally, there is only one additional isomorphism class V Γ 1,1 ⊗ V where V is a self-dual VOA of c = 24 without currents -it is the case where V = V ♮ is the Monster VOA [30]. From each of these isomorphism classes of vertex algebras, one can define a BKM algebra using Borcherds' construction based on chiral bosonic string theory compactified on V Γ 1,1 ⊗ V ; see [9,10,41,42,22,51] for explicit descriptions and denominator identities. 12 It is natural to conjecture that by tensoring any self-dual VOA of c = 24 with the bosonic (non-chiral) CFT on a circle S 1 , one gets one of precisely 12 distinct CFTs of left and right central charges (25,1).…”