  We study the effect of a possible helicity component of a primordial magnetic
field on the tensor part of the cosmic microwave background temperature
anisotropies and polarization. We give analytical approximations for the tensor
contributions induced by helicity, discussing their amplitude and spectral
index in dependence of the power spectrum of the primordial magnetic field. We
find that an helical magnetic field creates a parity odd component of gravity
waves inducing parity odd polarization signals. However, only if the magnetic
field is close to scale invariant and if its helical part is close to maximal,
the effect is sufficiently large to be observable. We also discuss the
implications of causality on the magnetic field spectrum.
