The Red Rectangle as a peculiar evolved binary

Since the discovery paper, back in 1975, the Red Rectangle nebula around its central bright object HD 44179, has remained a very popular object of study over a wide wavelength domain. Its former role as proto-typical example of a proto-planetary nebula has changed in the recent years, and the object is now understood as an evolved object in a binary orbit. The binary is not in contact any longer, but during the former cool giant phase, interaction must have been severe. The inner structure of the object is a binary with a period of 318 +/- 3 days in an eccentric orbit (e = 0.37), surrounded by a stable circumbinary disc in Keplerian rotation. This presence of a disc is suspected in many other post-AGB binaries, but it is the circumstellar chemistry and the nebular morphology that makes the Red-Rectangle unique.