  We present the discovery of the first T dwarf + white dwarf binary system
LSPM 1459+0857AB, confirmed through common proper motion and spectroscopy. The
white dwarf is a high proper motion object from the LSPM catalogue that we
confirm spectroscopically to be a relatively cool (Teff=5535+\-45K) and
magnetic (B~2MG) hydrogen-rich white dwarf, with an age of at least 4.8Gyrs.
The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey (ULAS
1459+0857), and has a spectral type of T4.5+\-0.5 and a distance in the range
43-69pc. With an age constraint (inferred from the white dwarf) of >4.8Gyrs we
estimate Teff=1200-1500K and logg=5.4-5.5 for ULAS 1459+0857, making it a
benchmark T dwarf with well constrained surface gravity. We also compare the T
dwarf spectra with the latest LYON group atmospheric model predictions, which
despite some shortcomings are in general agreement with the observed properties
of ULAS 1459+0857. The separation of the binary components (16,500-26,500AU, or
365 arcseconds on the sky) is consistent with an evolved version of the more
common brown dwarf + main-sequence binary systems now known, and although the
system has a wide separation, it is shown to be statistically robust as a non
spurious association. The observed colours of the T dwarf show that it is
relatively bright in the z band compared to other T dwarfs of similar type, and
further investigation is warranted to explore the possibility that this could
be a more generic indicator of older T dwarfs. Future observations of this
binary system will provide even stronger constraints on the T dwarf properties,
and additional systems will combine to give a more comprehensively robust test
of the model atmospheres in this temperature regime.
