X-ray Binaries (XRBs)
My main research interest centres on XRBs, of which there are various classes. One of the binary pair is a stellar remnant (either a black hole or a neutron star) accreting matter from a fusion-powered binary companion and emitting X-rays as it does so.
These systems are often said to provide remote laboratories for the study of the behaviour of matter in extreme gravitational fields and for the testing of general relativity.
A related class that is of interest to me are cataclysmic variables – binaries where the degenerate component is a white dwarf.