Constraining $(\Omega,\Lambda)$ from weak lensing in clusters: the   triplet method
  We present a new geometrical method which uses weak gravitational lensing effects in clusters to constrain the cosmological parameters Omega and Lambda. On each background galaxy, a cluster induces a convergence and a shear terms, which depend on the cluster projected potential, and on the cosmological parameters (Omega,Lambda), through the angular distance ratio D_{LS}/D_{OS}. To disentangle the effects of these three quantities, we compare the relative values of the ellipticities for each triplet of galaxies located at about the same position in the lens plane, but having different color redshifts. The simultaneous knowledge of the ellipticities and photometric redshifts enable to build a purely geometrical estimator of (Omega,Lambda) which does not depend on the lens potential.A calculation and comparison of each source of statistical noise is performed. We also analyzed the systematics.Simulations lead to encouraging constraints on the curvature of the Universe and can discriminate an open from a flat Universe. The test can also provide constraints on Lambda. By using 20 VLT nights to observe 100 clusters, we can reach Lambda_{-0.2}^{+0.3} for \Omega+\Lambda=1, or Omega_{-0.25}^{+0.3} for \Lambda=0. The method is even better adapted to a large program on the NGST. 
L. Gautret(1), B. Fort (2), Y. Mellier (1,2), ((1) Obs. de Paris,   DEMIRM. (2) IAP)