  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
$n$.
