2025ApJ...994..121W

Much of the carbonaceous dust observed in the early Universe may originate from colliding wind binaries (CWBs) hosting hot, luminous Wolf-Rayet (W-R) stars. Downstream of the shock between the stellar winds there exists a suitable environment for dust grain formation, and the orbital motions of the stars wrap this dust into richly structured spiral geometries. The Apep system is the most extreme W-R CWB in our Milky Way: two W-R stars produce a complex spiral dust nebula, whose slow expansion has been linked to a gamma-ray burst progenitor. It has been unclear whether the O-type supergiant 0$\mathop{.}\limits^{\unicode{x02033}}$7 distant from the W-R + W-R binary is physically associated with the system, and whether it affects the dusty nebula. Multiepoch Very Large Telescope/VISIR and JWST/MIRI observations show that this northern companion star routinely carves a cavity in the dust nebula-the first time such an effect has been observed in a CWB-which unambiguously associates the O star as a bound component to the Apep system. These observations are used together with a new geometric model to infer the cavity geometry and the orbit of the W-R + W-R binary, yielding the first strong constraints on wind and orbital parameters. We confirm an orbital period of over 190 yr for the inner binary-nearly an order of magnitude longer than the next longest-period dust-producing W-R CWB. This, together with the confirmed classification as a hierarchical triple, cements Apep as a singular astrophysical laboratory for studying colliding winds and the terminal life stages of the most massive star systems.