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We present an analysis of over 150 ks of data on the planetary nebula WeBo 1(PN G135.6+01.0) obtained with the Swift Ultraviolet Optical Telescope (UVOT).The central object of this nebula has previously been described as a late-typeK giant barium star with a possible hot companion, most likely a youngpre-white dwarf. UVOT photometry shows that while the optical photometry isconsistent with a large cool object, the near-ultraviolet (UV) photometry showsfar more UV flux than could be produced by any late-type object. Using modelstellar atmospheres and a comparison to UVOT photometry for the pre-white dwarfPG 1159-035, we find that the companion has a temperature of at least 40,000 Kand a radius of, at most, 0.056 R_sun. While the temperature and radius areconsistent with a hot compact stellar remnant, they are lower and larger,respectively, than expected for a typical young pre-white dwarf. This likelyindicates a deficiency in the assumed UV extinction curve. We find that highertemperatures more consistent with expectations for a pre-white dwarf can bederived if the foreground dust has a strong "blue bump" at 2175 AA and a lowerR_V. Our results demonstrate the ability of Swift to both uncover andcharacterize hot hidden companion stars and to constrain the UV extinctionproperties of foreground dust based solely on UVOT photometry.

Swift/UVOT Photometry of the Planetary Nebula WeBo 1: Unmasking A Faint Hot Companion Star