Bipolar nebulae and binary stars : the family of crabs He 2-104, BI Crucis and MyCn 18.

We present CCD images and long slit spectra of the symbiotic bipolar nebulae He 2-104 and BI Crucis and of a young PN, MyCn 18. He 2-104 and BI Cru show extended bipolar nebulae and collimated jets expanding at very high velocities. In He 2-104, the presence of a second inner bipolar nebula provides evidence for multiple ejection events. Using these data, the energy distribution and information from the literature, we discuss the nature of these objects and conclude that their nebulae and jets are collimated by a binary system containing an accretion disk. We further conclude that He 2-104 and BI Cru are similar objects at different evolutionary stages but not genuine PNe. In fact, they are systems in which the two central stars are respectively in a pre-PN phase (the cool component) and in a post-PN phase (the hot component), with the old PN already having dispersed in the ISM. The case of MyCn 18 is more uncertain; apart from its striking morphological similarity to He 2-104, there are no other strong indications supporting the idea that it could belong to the same family of post-PN symbiotic nebulae as He 2-104 and BI Cru.