Infrared renormalons and power suppressed effects in e+e- jet events

We study the effect of infrared renormalons upon shape variables that are commonly used to determine the strong coupling constant in e^+e^- annihilation into hadronic jets. We consider the model of QCD in the limit of large n_f. We find a wide variety of different behaviours of shape variables with respect to power suppressed effects induced by infrared renormalons. In particular, we find that oblateness is affected by 1/Q non--perturbative effects even away from the two jet region, and the energy--energy correlation is affected by 1/Q non--perturbative effects for all values of the angle. On the contrary, variables like thrust, the c parameter, the heavy jet mass, and others, do not develop any 1/Q correction away from the two jet region at the leading n_f level. We argue that 1/Q corrections will eventually arise at subleading n_f level, but that they could maintain an extra \as(Q) suppression. We conjecture therefore that the leading power correction to shape variables will have in general the form \alpha^n_{\rm S}(Q)/Q, and it may therefore be possible to classify shape variables according to the value of n.