  We report the discovery and analysis of a massive, compact, hierarchical
triple system (TIC 470710327) initially identified by citizen scientists in
data obtained by NASA's Transiting Exoplanet Survey Satellite (TESS).
Spectroscopic follow-up observations obtained with the HERMES spectrograph,
combined with eclipse timing variations (ETVs), confirm that the system is
comprised of three OB stars, with a compact 1.10 d eclipsing binary and a
non-eclipsing tertiary on a 52.04 d orbit. Dynamical modelling of the system
(from radial velocity and ETVs) reveal a rare configuration wherein the
tertiary star (O9.5-B0.5V; 14-17 M$_{\odot}$) is more massive than the combined
mass of the inner binary (10.9-13.2 M$_{\odot}$). Given the high mass of the
tertiary, we predict that this system will undergo multiple phases of mass
transfer in the future, and likely end up as a double neutron star
gravitational wave progenitor or an exotic Thorne-Zytkow object. Further
observational characterisation of this system promises constraints on both
formation scenarios of massive stars as well as their exotic evolutionary
end-products.
