Binary white dwarfs in the halo of the Milky Way Pim van Oirschot, Gijs Nelemans, Silvia Toonen, Onno Pols, Anthony G. A. Brown, Amina Helmi and Simon Portegies Zwart A&A, 569 (2014) A42 Published online: 16 September 2014 DOI: 10.1051/0004-6361/201424195