Gravitational Lenses and the Hubble Constant : Present and Future
  The best studied gravitational lens is the Twin QSO Q0957+561, and as a result of recent advances in observations and modelling, a ten percent measurement of the Hubble constant is attainable. We discuss in detail this system and review the perspectives in a near future. 
L.J. Goicoechea (Univ. Cantabria, Spain), E. Mediavilla (IAC, Spain),   A. Oscoz (IAC, Spain), M. Serra-Ricart (IAC, Spain) ...