Binaries as Sources of Gravitational Waves

With the discovery of both binary black hole mergers and a binary neutron star merger the field of Gravitational Wave Astrophysics has really begun. The current advanced LIGO and Virgo detectors are laser interferometers that will improve their sensitivity in the coming years. In the long run, new detectors such as LISA and the Einstein Telescope will have sensitivities that allow the detection of many thousands of sources and ET can observe essentially the whole observable Universe, for heavy black holes. All these measurements will provide new answers to open questions in binary evolution, related to mass transfer, out-of-equilibrium stars and the role of metallicity. In addition, the data will give new constraints on uncertainties in the evolution of (massive) stars, such as stellar winds, the role of rotation and the final collapse to a neutron star or black hole. For black hole binaries, the number of detections is rapidly approaching 10 and the first proper statistical studies of the population can be done soon. In the long run, the thousands of detections by ET will enable us to probe their population in great detail over the history of the Universe. For neutron stars, the first question is whether the first detection GW170817 is a typical source or not. In any case, it has spectacularly shown the promise of complementary electro-magnetic follow-up. For white dwarfs we have to wait for LISA (around 2034) but new detections by e.g. Gaia and LSST will prepare for the astrophysical exploitation of the LISA