The companion could be a main sequence star, white dwarf, neutron star or black hole.
We detected pulsar-pulsar binaryhere several decades ago. But if one of the pulsar does not radiate towards us, we may be not able to tell it is a pulsar or a black hole.
If the companion is not a main sequence star, how to know the nature of the companion in a neutron star binary?
Of the binary systems with one single neutron star, is that possible there is another neutron star or a hidden black hole?