Have we indirectly measured GW driven orbital decay in anything besides binary pulsars?

Question

Binary systems decay for all sorts of reasons. I'm asking specifically about orbital decay due to gravitational wave emission.

All of direct GW detections of binary systems have come with orbit change measurements (the GW frequency chirp), but that's not what I'm asking about.

Binary pulsar systems, usually binary neutron star or neutron star-white dwarf, are great for precise orbital measurements, and in a handful of these systems folks have measured the change in the system's orbital period $\dot{P}_B$ due to gravitational wave emission.

• Are there any other systems where GW driven orbital decay has been observed?
• Is it possible to optically observe a tight double white dwarf and get that kind of orbital evolution precision?
• What about galactic center objects orbiting Sag A$^*$? Are the dynamics modeled as Keplerian orbits? Do any of closer encounters require post-Keplerian corrections like $\dot{P}_B$?

Answer 1

Are there any other systems where GW driven orbital decay has been observed?... Is it possible to optically observe a tight double white dwarf and get that kind of orbital evolution precision?

Yes. The first double white dwarf binary was observed in the late 1980s. The ESA's space-based gravitational-wave observatory, LISA, is highly anticipated to detect gravitational waves from these systems. For example, see this paper about the double white-dwarf binary PTF J053332.05+020911.6 which has an orbital period of $\approx 20$ minutes. From their abstract:

Because this system already has a well-determined orbital period, radial velocity semi-amplitude, temperature, atmospheric composition, surface gravity, and orbital decay rate, a LISA signal will help fully constrain the properties of this system by providing a direct measurement of its inclination. The mHz band of LISA will be dominated by compact binaries with orbital periods less than hours.

Also see this study about an $\approx 8$ minute and this study about an $\approx 7$ minute period double white dwarf binary, respectively.

As far as I can see from my quick search, it is typical (e.g., see this, this, this, and this) to consider the double white dwarf binary in a circular orbit for galactic/cosmological scale population synthesis and hydrodynamical simulations. However, post-Newtonian corrections are important when one is concerned about the gravitational waveform, for example when measuring the individual masses of the white dwarves.