The discovery of GW170817 with gravitational waves (GWs) and electromagnetic
(EM) radiation is prompting new questions in strong-gravity astrophysics.
Importantly, it remains unknown whether the progenitor of the merger comprised
two neutron stars (NSs), or a NS and a black hole (BH). Using new
numerical-relativity simulations and incorporating modeling uncertainties we
produce novel GW and EM observables for NS-BH mergers with similar masses. A
joint analysis of GW and EM measurements reveals that if GW170817 is a NS-BH
merger, <40% of the GW parameters are compatible with EM observations.