  We investigate the observational consequences of the light-like deformations
of the Poincar\'e algebra induced by the jordanian and the extended jordanian
classes of Drinfel'd twists. Twist-deformed generators belonging to a Universal
Enveloping Algebra close nonlinear algebras. In some cases the nonlinear
algebra is responsible for the existence of bounded domains of the deformed
generators. The Hopf algebra coproduct implies associative nonlinear additivity
of the multi-particle states. A subalgebra of twist-deformed observables is
recovered whenever the twist-deformed generators are either hermitian or
pseudo-hermitian with respect to a common invertible hermitian operator.
