I derive the kinematical variable mT2 as a generalization of the mT variable, used at the CERN UA1 experiment to measure the W mass from the semi-invisible mode W $to$ e nu. I then discuss a number of properties of mT2, that make it a very attractive tool for LHC events. For example its “kink”, allowing to determine two new-particles masses simultaneously. Finally, I illustrate the above ideas in a concrete example, taken from SUSY GUTs. centerline{Note: UNUSUAL DATE and PLACE}

LPT Orsay