Per the classical, textbook definition, planetary nebulae are the glowing shells of gas and dust expelled by single, Sun-like stars at the end of their lives as they evolve towards the white dwarf phase. However, there is growing evidence that a significant fraction, perhaps even a majority, of planetary nebulae are the result of binary interactions (with some estimates as high as 80%). In particular, it is now clear that at least 20% are the product of the ever-so-poorly understood common-envelope phase of binary stellar evolution critical in the formation of a plethora of astrophysical phenomena, ranging from stellar-mass gravitational wave sources through to the cosmologically-important type Ia supernovae. In spite of its now obvious importance, the exact role that the common-envelope phase plays in the formation and evolution of planetary nebulae remains elusive, principally due to the relative paucity of known systems which has, until now, plagued the field. As such, the time is right for a coordinated observational and theoretical campaign to study both the central stars and their host nebulae, in order to drive towards finally understanding the common-envelope phase. Critically, this work will have wider impacts beyond the understanding of planetary nebulae, providing valuable insights into close-binary evolution in general. For example, by constraining the common envelope efficiency - the holy grail of close-binary population synthesis studies - and the possible binary origins of the abundance discrepancy problem in photo-ionised nebulae, an issue which has remained unsolved since its discovery nearly 80 years ago.

In this project, we propose to characterise for a significant number of post-common-envelope planetary nebulae:

-The physical and orbital properties of their central stars

-The morpho-kinematical properties of the nebular ejecta

-The chemical abundance and physical conditions of the nebular ejecta, with particular emphasis on the spatial distribution of the two gas phases which are thought to be the cause of their extreme abundance discrepancies.

With this information, we will be able to begin to construct a self-consistent picture of the common-envelope process. We will constrain the conditions for a common-envelope phase to occur, and for the envelope to be successfully ejected, as well as the impact the phase has on the properties of the stars and the ejecta.