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SuWt2 is a planetary nebula (PN) consisting of a bright ionized thin ringseen nearly edge-on. It has a bright (V=12) central star, too cool to ionizethe PN, which we discovered to be an eclipsing binary. A spectrum from IUE didnot reveal a UV source. We present extensive ground-based photometry andspectroscopy of the central binary collected over the ensuing two decades,resulting in the determination that the orbital period of the eclipsing pair is4.9 d, and consists of two nearly identical A1 V stars, each of mass ~2.7M_sun. The physical parameters of the A stars, combined with evolutionarytracks, show that both are in the short-lived "blue-hook" evolutionary phasethat occurs between the main sequence and the Hertzsprung gap, and that the ageof the system is about 520 Myr. One puzzle is that the stars' rotationalvelocities are different from each other, and considerably slower thansynchronous with the orbital period. It is possible that the center-of-massvelocity of the eclipsing pair is varying with time, suggesting that there isan unseen third orbiting body in the system. We propose a scenario in which thesystem began as a hierarchical triple, consisting of a ~2.9 M_sun star orbitingthe close pair of A stars. Upon reaching the AGB stage, the primary engulfedthe pair into a common envelope, leading to a rapid contraction of the orbitand catastrophic ejection of the envelope into the orbital plane. In thispicture, the exposed core of the initial primary is now a white dwarf of ~0.7M_sun, orbiting the eclipsing pair, which has already cooled below thedetectability possible by IUE at our derived distance of 2.3 kpc and areddening of E(B-V)=0.40. The SuWt2 system may be destined to perish as a TypeIa supernova. (Abridged)

The Exotic Eclipsing Nucleus of the Ring Planetary Nebula SuWt2