  The new contributions to the electron (muon) anomalous magnetic moment
arising in mirror fermion theories have been calculated. Imposing the
experimental constraint lowers the current upper bound on the ordinary - mirror
lepton mixing angles by a factor of 50 making predictions for mirror lepton
production at HERA undetectably small. A way out is to allow for different
mixing angles of the L and R field components. Choosing very small right mixing
angles compatibility with the anomalous magnetic moment measurement may be
easily maintained, while choosing left mixing angles close to the upper limits
yields still reasonable HERA cross-sections.
