  We present the discovery of the eclipsing double white dwarf (WD) binary WDJ
022558.21-692025.38 that has an orbital period of 47.19 min. Following
identification with the Transiting Exoplanet Survey Satellite, we obtained
time-series ground based spectroscopy and high-speed multi-band ULTRACAM
photometry which indicate a primary DA WD of mass 0.40 +- 0.04 Msol and a 0.28
+- 0.02 Msol mass secondary WD, which is likely of type DA as well. The system
becomes the third-closest eclipsing double WD binary discovered with a distance
of approximately 400 pc and will be a detectable source for upcoming
gravitational wave detectors in the mHz frequency range. Its orbital decay will
be measurable photometrically within 10 yrs to a precision of better than 1%.
The fate of the binary is to merge in approximately 41 Myr, likely forming a
single, more massive WD.
