  A new statistic is briefly reviewed, designed to detect isolated coherent
step-like discontinuities produced by cosmic strings present at late times. As
a background I superpose a scale invariant Gaussian random field which could
have been produced by a superposition of seeds on all scales and/or by
inflationary quantum fluctuations. The statistical variable considered is the
Sample Mean Difference (SMD) between large neighbouring sectors of CMB maps,
separated by lines in two dimensional maps and points in one dimensional maps.
I find that the SMD statistics can detect at the $1\sigma$ level the presense
of a long string with $G\mu (v_s \gamma_s)={1\over {8\pi}}({{\delta T}\over
T})_{rms} \simeq 0.5 \times 10^{-7}$ while more conventional statistics like
the skewness or the kurtosis require a value of $G\mu$ almost an order of
magnitude larger for detectability at a comparable level.
