clearly shows that, despite the lack of an appropriate model, traders have empirically adapted to incorporate some subtle information on the real statistics of price changes. More precisely, a `call' option is such that if the price x(T ) of a given asset at time T (the `maturity') exceeds a certain level x c (the `strike' price), the owner of the option receives the difference x(T ) Γ x c . Conversely, if x(T ) ! x c , the contract is lost. To make a long story short [1, 2, 3], if T is...