We present spectra of the DAB white dwarf PG 1115+166. Radial velocity measurements of the Balmer lines and the He i 6678 Å line show that this is a binary white dwarf with a period of 30.09 d in which the Balmer lines move in anti-phase to the He i line, i.e. PG 1115+166 is a DA+DB binary. The minimum masses of the stars are MDA= 0.43 ± 0.15 M⊙ and MDB= 0.52 ± 0.12 M⊙. The separation of the stars is about 45 R⊙, which is much smaller than a typical asymptotic giant branch (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. PG 1115+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 a long orbital period and high mass found in PG 1115+166. We conclude that PG 1115+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 PG 1115+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
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