Graphs of groups and the Atiyah conjecture for one-relator groups
  Paper withdrawn because of a gap in the proof of Proposition 3 of Thomas Schick: "Integrality of L2-Betti numbers", Math. Ann. 317, 727-750 (arXiv.org/abs/math.gt/0001101). Most results of the withdrawn paper were based on this proposition. 
Warren Dicks (UA Barcelona) and Thomas Schick (Uni Muenster)