Classical solutions for exotic instantons?

Abstract

We consider the D7/D(–1) system in Type I' as a prototypical ``exotic'' brane instanton. With respect to systems such as the D3/D(–1) ones, which correspond to gauge instantons in four dimensions, exotic systems lack the bosonic mixed moduli w of the ADHM construction, related to the instanton size ρ, and their possible field-theoretical interpretation as classical solutions is an important open question. For the system at hand, we propose that it corresponds to the point-like limit ρ → 0 of the eight-dimensional so-called SO(8) instanton solution. This configuration is a solution of the quartic term of the non-abelian D7 action, i.e. the term which stays finite in the limit α' → 0 with gs fixed that preserves the D(–1) effects. As a necessary consistency condition, we check that the next order term in the non-abelian effective action vanishes on the SO(8) solution so that the limit we take is well-defined.