The objective of the so-called fringe sensor DWARF - derived from DARWIN AstRonomical Fringe Sensor is to measure all relevant perturbations and to provide real time control data to achieve co-phasing of the freeflying telescopes of the DARWIN system. An overview of the design of the sensor Breadboard (BB), as it has been designed and built at Kayser-Threde as Prime with main partners ONERA and Alcatel Space under ESA contract 17012/03/NL/EC is presented. An extensive test campaign has been carried out at ONERA. The respective optical test setup is outlined and the achieved performances for retrieval of primary and higher order aberrations are presented. As final outcome of the BB study the essential elements to build a fringe sensor (FS) flight model are shortly characterized and the respective development roadmap is sketched.