  In their recent paper ``Faraday rotation of the cosmic microwave background
polarization by a stochastic magnetic field'', Kosowsky et al. Phys.Rev. D71,
043006 (2005) have commented about our paper [C.Caprini and R.Durrer, Phys.
Rev. D65, 023517 (2002)], in which we derived very strong limits on the
amplitude of a primordial magnetic field from gravitational wave production.
They argue that our limits are erroneous. In this short comment we defend our
result.
