  Both the red giants and the Mira variables in symbiotic systems have
systematically higher mass-loss rates than do typical galactic giants and
Miras, which suggests that only very evolved giants, and so those with highest
mass-loss rates, can support symbiotic behaviour in widely separated binary
systems. They often show a flattened mass-loss geometry due to an intrinsically
inhomogeneous mass loss and/or tidal interactions between the binary
components. The main body of a symbiotic nebula is thus formed from material
lost in the giant wind, while the hot component is responsible for its
ionization and excitation. In addition, the fast wind and/or jet ejection from
the hot component, whenever occur, give rise to the complex, often bipolar,
shape of symbiotic nebulae. Observations of resolved nebulae also suggest that
the binary geometry and nebular structure are aligned but the bipolar outflow
may be not orthogonal to the orbital plane in all cases.
