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Published online by Cambridge University Press: 26 May 2016
Recently, high-resolution imaging surveys of young planetary nebulae (PNe) and protoplanetary nebulae (PPNe) have revealed that the majority of these objects are characterised by multipolar bubbles distributed roughly point-symmetrically around the central star (e.g. Sahai & Trauger 1998, Sahai 2000). Sahai & Trauger (1998) have proposed that episodic high-speed jet-like outflows, operating during the protoplanetary or very late-AGB phase, are the primary agent for shaping PNe. OH and H2O masers provide a unique and crucial probe of the kinematics of the circumstellar material in PPNe, because of the general lack of other emission-line diagnostics. Here we present new results from our ongoing study of PPNe using HST images with interferometric OH & H2O maser-line data to unravel their detailed spatio-kinematic structure (e.g. Sahai et al. 1999a, Sahai, Claussen, & Morris 2002).