  We present spectra of the DAB white dwarf PG1115+166. Radial velocity
measurements of the Balmer lines and the HeI6678 line show that this is a
binary white dwarf with a period of 30.09d in which the Balmer lines move in
anti-phase to the HeI line, i.e., PG1115+166 is a DA+DB binary. The minimum
masses of the stars are M_DA=0.43+-0.15 solar masses and M_DB=0.52+-0.12 solar
masses. The separation of the stars is about 45 solar radii which is much
smaller than a typical AGB progenitor of a white dwarf, implying that there has
been at least one common envelope phase in this binary. Indeed, it is possible
that this binary may have suffered up to three mass transfer episodes - two
associated with the red giant phase prior to the formation of each white dwarf
and a third associated with the ``born-again'' red giant phase of the DB white
dwarf. PG1115+166 has the longest orbital period of any post common envelope
white dwarf - white dwarf binary found to date. Published models for the
formation of white dwarf - white dwarf binaries do not predict any white dwarfs
with the combination of long orbital period and high mass found in PG1115+166.
We conclude that PG1115+166 is a key object for testing models of binary star
evolution and it may also be a key object for our understanding of the
formation of DB white dwarfs. We outline the observational tests which can be
applied to scenarios for the formation of PG1115+166 and apply them to the
simplest case of a single common envelope phase. This suggests that some part
of the internal energy stored in the envelope of the AGB star, e.g., as ionized
hydrogen, may have contributed to the ejection of the common envelope, but
there are several unanswered questions concerning this simple scenario.
