[shagie]

The next problem is https://arxiv.org/pdf/1006.2834.pdf which has the lightest black holes at 5.0⊙

There's another interesting object - https://www.science.org/doi/10.1126/science.adg3005

This is a binary system with a total mass of 3.887⊙ ± 0.004 ... which puts the companion in the system at 2.09⊙ to 2.71⊙ ... which may be in the "heavier than any theoretical neutron star, but lighter than any known black hole.

A recent Dr. Becky video on this: FOUND in the MASS GAP: The heaviest neutron star OR the lightest black hole? https://youtu.be/8PHt7NcwllA