Visible spectroscopy of the two largest known trans-Neptunian objects: Ixion and Quaoar

We report visible spectroscopy of the two largest known trans-Neptunian objects (TNOs): Quaoar (=50 000) and Ixion (=28 976) obtained respectively on 4th and 5th May 2003 with the ESO New Technology Telescope (NTT) at La Silla, Chile. The spectrum of Ixion is moderately red with an absorption feature around 0.80 (mu)m, while Quaoar is a red object with a featureless spectrum. These differences seem to indicate differences in primordial composition or dynamical history, in spite of their present similar orbits.