2000A&A...360.1011N

The unique core-mass - radius relation for giants with degenerate helium cores enables us to reconstruct the evolution of three observed double helium white dwarfs with known masses of both components. The last mass transfer phase in their evolution must have been a spiral-in. In the formalism proposed by (Webbink, 1984), we can constrain the efficiency of the deposition of orbital energy into the envelope to be 1 ≲α≲6, for an envelope structure parameter λ=0.5. We find that the two standard mass transfer types (stable mass transfer and spiral-in) are both unable to explain the first phase of mass transfer for these three binaries. We use a parametric approach to describe mass transfer in low-mass binaries, where both stars are of comparable mass and find that the orbital characteristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with ∼1.5 times the specific angular momentum of the initial binary. In this case no substantial spiral-in occurs.