Gravitational Lensing and Tests of the Cosmological Constant
  The case for a flat Cold Dark Matter model with a positive cosmological constant $\Lambda$ has been recently strongly advocated by some theoreticians. In this paper we give the observers point of view to the light of the most recent observations with a special emphasis on lensing tests. We confirm the apparent cosmic concordance for a flat Universe with $\Omega_{\Lambda}$ close to 0.6 but we note that a low mass density open universe with no cosmological constant is still quite acceptable for most of the reliable observational tests, including lensing tests as well. 
B. Fort (1,2), Y. Mellier (1,2) ((1) IAP; (2) Obs. Paris DEMIRM)