We show that the B-mode polarization signal detected at low multipoles by BICEP2 can not be entirely due to topological defects .
This would be incompatible with the high-multipole B-mode polarization data and also with existing temperature anisotropy data .
Adding cosmic strings to a model with tensors , we find that B-modes on their own provide a comparable limit on the defects to that already coming from Planck satellite temperature data .
We note that strings at this limit give a modest improvement to the best-fit of the B-mode data , at a somewhat lower tensor-to-scalar ratio of r \simeq 0.15 .