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Constraining the properties of K_{l3} form factors with analyticity and unitarity

Bounds on the shape parameters of K_{l3} form factors are investigated in a general model independent framework. Using the method of unitarity bounds for correlators evaluated in pQCD, chiral symmetry and experimental information on the phase and modulus of πK form factors up to an energy t_{in}=1 GeV^2, we constrain the shape parameters of the scalar and vector form factors, the value of the scalar form factor at t=M_π^2-M_K^2, and exclude regions where zeros can exist on the real energy line and in the complex energy plane.