  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.
