  In the case of the process $e^+e^-\to\mu^+\mu^-$, we study the sensitivity to
either $Z^\prime$ or anomalous gauge coupling effects of two new observables,
$\sigma_+$ and $\sigma_-$, conveniently defined for such kind of analysis. We
discuss general properties of the deviations of these observables from the
Standard Model predictions, and derive the model-independent bounds on the
relevant parameters allowed by present and foreseeable LEP2 data. We also
discuss the possibility of separating the two kinds of effect.
