  String models can produce successful inflationary scenarios in the context of
brane collisions and in many of these models cosmic strings may also be
produced. In scenarios such as KKLMMT the string contribution is naturally
predicted to be well below the inflationary signal for cosmic microwave
background (CMB) temperature anisotropies, in agreement with the existing
limits. We find that for $B$ type polarization of CMB the situation is reversed
and the dominant signal comes from vector modes generated by cosmic strings,
which exceeds the gravity wave signal from both inflation and strings. The
signal can be detected for a broad range of parameter space: future
polarization experiments may be able to detect the string signal down to the
string tension $G\mu=10^{-9}$, although foregrounds and lensing are likely to
worsen these limits. We argue that the optimal scale to search for the string
signature is at $\ell\sim 1000$, but in models with high optical depth the
signal from reionization peak at large scales is also significant. The shape of
the power spectrum allows one to distinguish the string signature from the
gravity waves from inflation, but only with a sufficiently high angular
resolution experiment
