Reconstructing the evolution of double helium white dwarfs: envelope loss without spiral-in

The unique core-mass - radius relation for giants with degenerate helium co res 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 efficien cy of the deposition of orbital energy into the envelope to be 1 less than or similar to alpha less than or similar to 6, for an envelope structure pa rameter lambda = 0.5. We find that the two standard mass transfer types (st able mass transfer and spiral-in) are both unable to explain the first phas e 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 charact eristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with similar to 1.5 times the specif ic angular momentum of the initial binary. In this case no substantial spir al-in occurs.