Binary stars as the key to understanding planetary nebulae
  Planetary nebulae are traditionally considered to represent the final
evolutionary stage of all intermediate-mass stars ($\sim$0.7-8Msol). Recent
evidence seems to contradict this picture. In particular, since the launch of
the Hubble Space Telescope it has become clear that planetary nebulae display a
wide range of striking morphologies which cannot be understood in a single star
scenario, instead pointing towards a binary evolution in a majority of systems.
Here, we summarise our current understanding of the importance of binarity in
the formation and shaping of planetary nebulae, as well as the surprises that
recent observational studies have revealed with respect to our understanding of
binary evolution in general. These advances have critical implications,
including for the understanding of mass transfer processes in binary stars -
particularly the all-important but ever-so poorly understood `common envelope
phase' - as well as the formation of cosmologically important type Ia
supernovae.
