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The Solar Neighborhood XXXI: Discovery of an Unusual Red+White Dwarf Binary at ~25 Parsecs via Astrometry and UV Imaging

We report the discovery of a nearby M5.0V dwarf at 24.6 pc, SCR 1848-6855, that is orbited by an unusual companion causing an astrometric perturbation of more than 200 milli-arcseconds. This is by far the largest perturbation found to date among more than 700 targets observed during our long-term astrometry/photometry program at the CTIO 0.9-m telescope. We present here a suite of astrometric, photometric, and spectroscopic observations of this high proper motion (~1.3"/yr) system in an effort to reveal the nature of this unusual binary. The measured near-UV and optical U band fluxes exceed those expected for comparable M5.0V stars, and excess flux is also detected in the spectral range 4000A--7000A. The elusive companion has been detected in HST-STIS+MAMA images at 1820A and 2700A, and our analysis shows that it is probably a rare, cool, white dwarf with T = 4600-5500K. Given the long-term astrometric coverage, the prospects for an accurate mass determination are excellent, although as yet we can only provide limits on the unusual companion's mass.