The "Twin Jet" Planetary Nebula M2-9
  We present a model for the structure, temporal behavior, and evolutionary status of the bipolar nebula M2-9. According to this model the system consists of an AGB or post-AGB star and a hot white dwarf companion, with an orbital period of about 120 years. The white dwarf has undergone a symbiotic nova eruption about 1200 years ago, followed by a supersoft x-ray source phase. The positional shift of the bright knots in the inner nebular lobes is explained in terms of a revolving ionizing source. We show that the interaction between the slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a path for the ionizing radiation in one direction, while the radiation is attenuated in others. This results in the mirror-symmetric (as opposed to the more common point-symmetric) shift in the knots. We show that M2-9 provides an important evolutionary link among planetary nebulae with binary central stars, symbiotic systems, and supersoft x-ray sources. 
Mario Livio (Space Telescope Sc. Ins.), Noam Soker (U. of Haifa)