Abstract
Binary stars are believed to be involved in some way in the formation of many collimated outflows observed in planetary nebulae. The direct detection of binary central stars is observationally difficult, so that indirect methods offer an interesting alternative for inferring the presence of a binary. It has been recently proposed that a difference between the systemic velocity of a collimated (bipolar) outflow and that of the main shell is direct evidence for orbital motion. We present and discuss this method and show data of Hu2-1 and IC4846, two planetary nebulae with multiple collimated components, in which the aforementioned systemic velocities clearly differ from each other. The orbital parameters estimated in Hu2-1 and IC4846 are typical of interacting binaries and suggest that the origin of the collimated outflows in these two planetary nebulae is an accretion disk around a companion.
