Data on muon pair production obtained by the OPAL Collaboration at centre o
f mass energies near the Z peak are analysed. Small angular mismatches betw
een the directions of the two muons are used to assess the effects of initi
al state photon radiation and initial-final-state radiation interference on
the forward-backward asymmetry of muon pairs. The dependence of the asymme
try on the invariant mass of the pair is measured in a model-independent wa
y. Effective vector and axial-vector couplings of the Z boson are determine
d and compared to the Standard Model expectations. (C) 2001 Elsevier Scienc
e B.V. All rights reserved.