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A kilonova signal is generally expected after a Black Hole - Neutron Starmerger. The strength of the signal is related to the equation of state ofneutron star matter and it increases with the stiffness of the latter. Therecent results obtained by NICER from the analyses of PSR J0740+6620 suggest arather stiff equation of state and the expected kilonova signal is thereforestrong, at least if the mass of the Black Hole does not exceed $\sim 10\mathrm{\,M}_\odot$ and if the adimensional spin parameter is not too small andthe orbit is prograde. We compare the predictions obtained by consideringequations of state of neutron star matter satisfying the most recentobservations and assuming that only one family of compact stars exists with theresults predicted in the two-families scenario. In the latter a soft hadronicequation of state produces very compact stellar objects while a rather stiffquark matter equation of state produces massive strange quark stars, satisfyingNICER results. The expected kilonova signal in the two-families scenario isvery weak: in particular the Hadronic Star - Black Hole merger produces a muchweaker signal than in the one-family scenario because the hadronic equation ofstate is very soft. Moreover, according to the only existing simulation, theStrange Quark Star - Black Hole merger does not produce a kilonova signalbecause the amount of mass ejected is negligible. These predictions will beeasily tested with the new generation of detectors if Black Holes with anadimensional spin parameter $\chi_\mathrm{BH}\gtrsim 0.2$ or a mass $\mathrmM_\mathrm{BH} \lesssim 4 \mathrm M_\odot$ can be present in the merger.

Merger of a Neutron Star with a Black Hole: one-family vs. two-families scenario