Binary post-AGB stars as tracers of stellar evolution

In this chapter the focus is on the properties of post-Asymptotic Giant Branch (post-AGB) stars in binary systems. Their Spectral Energy Distributions (SEDs) are very characteristic: they show a near-infrared excess, indicative of the presence of warm dust, while the central stars are too hot to be in a dust-production evolutionary phase. This allows for an efficient detection of binary post-AGB candidates. It is now well established that the near-infrared excess is produced by the inner rim of a stable dusty disc that surrounds the binary system. These discs are scaled-up versions of protoplanetary discs and form a second generation of stable Keplerian discs. They are likely formed during a binary interaction process when the primary was on ascending the AGB. I will summarise what we have learned from the observational properties of these post-AGB binaries. The impact of the creation, lifetime and evolution of the circumbinary discs on the evolution of the system are yet to be fully understood.