  The tensor contribution to the $V$-mode polarization induced by a magnetized
plasma at last scattering vanishes exactly. Conversely a polarized background
of relic gravitons cannot generate a $V$-mode polarization. The reported
results suggest that, in the magnetized $\Lambda$CDM paradigm, the dominant
source of circular dichroism stems from the large-scale fluctuations of the
spatial curvature.
