  In this letter we focus on the peculiar case of a coalescing compact-object
binary whose chirp mass is compatible both with a neutron star-neutron star and
black hole-neutron star system, with the black hole in the $\sim 3-5M_\odot$
range defined as the "mass gap". Some models of core-collapse supernovae
predict the formation of such low-mass black holes and a recent observation
seems to confirm their existence. Here we show that the nature of the companion
to the neutron star can be inferred from the properties of the kilonova
emission once we know the chirp mass, which is the best constrained parameter
inferred from the gravitational signal in low-latency searches. In particular,
we find that the kilonova in the black hole-neutron star case is far more
luminous than in the neutron star-neutron star case, even when the black hole
is non spinning. The difference in the kilonovae brightness arises primarily
from the mass ejected during the merger. Indeed, in the considered interval of
chirp masses, the mass ejection in double neutron star mergers is at its worst
as the system promptly forms a black hole. Instead mass ejection for black
hole-neutron star case is at its best as the neutron stars have low mass/large
deformability. The kilonovae from black hole-neutron star systems can differ by
two to three magnitudes. The outcome is only marginally dependent on the
equation of state. The difference is above the systematics in the modeling.
