  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.
