Both the black hole and the companion star in V1408 Aquilae circle around each other every 9.3 hours. Models indicate that the black hole most likely has 3 times the Sun’s mass. However, the mass of the black hole is not well constrained and can be as high as 6 times the Sun’s mass, with a 90 percent probability of not exceeding this value. Nevertheless, the black hole in V1408 Aquilae is still a good candidate for a compact object that lies within the mass gap of 2 to 4 times the Sun’s mass.
Gomez, Mason & Robinson (2015), “The Case for a Low Mass Black Hole in the LMXB V1408 Aquilae (4U 1957+115)”, American Astronomical Society, AAS Meeting #221, #142.25