Supermassive black holes locked in a stable orbit around each other

> The existence of two black holes in OJ287 was first suggested in 1982. Aimo Sillanpää, then a graduate student at the University of Turku, observed that the brightness of the quasar changed regularly over a 12-year cycle.

Damn, that's about the time it takes Jupiter to orbit the sun. That feels wildly close together for objects that mass 18 billion & 150 million times that of our own sun.

These black holes (according to a calculator I found online) have radii of 53 billion km and 400 million km, so I'm guessing they must be orbiting significantly further away, and significantly faster than Jupiter (which is ~800 million km away from the sun) - which makes sense, given the monstrous 18b figure. I wonder how far apart they are, but I don't really know how to easily calculate that right now.

  Feature                  Primary Black Hole              Secondary Black Hole
  -----------------------  ------------------------------  ------------------------------
  Mass                     1.8 × 10^10 M                   1.5 × 10^8 M
  Schwarzschild Radius     356 AU                          3.0 AU
 
  --- Circular / Average Orbital Properties ---
  Orbital Period           12 years
  Semi-Major Axis          13,800 AU (~0.22 ly)
  Orbital Speed (avg)      282 km/s (0.094% c)             33,900 km/s (11.3% c)

  --- Elliptical Orbit (e ≈ 0.65) ---
  Pericenter Distance      4,830 AU                        (same)
  Orbital Speed (peri)     613 km/s (0.20% c)              73,600 km/s (24.5% c)

  Apocenter Distance       22,800 AU                       (same)
  Orbital Speed (apo)      130 km/s (0.043% c)             15,600 km/s (5.2% c)

So the "smaller" SMBH is punching through the larger one's disk at a significant fraction of c twice every 12 years. But it's losing energy to gravitational waves so quickly that they'll probably merge in around 10,000 years [1]

[1] https://archive.is/Ccy5M