Quasar variability limits on cosmological density of cosmic strings

We put robust upper limits on the average cosmological density \Omega_s of cosmic strings based on the variability properties of a large homogeneous sample of SDSS quasars. We search for an excess of characteristic variations of quasar brightness that are associated with string lensing and use the observed distribution of this variation to constrain the density of strings. The limits obtained do not invoke any clustering of strings, apply to both open segments and closed loops of strings, usefully extend over a wide range of tensions 10^{-13} < G\mu/c^2 < 10^{-9} and reach down the level of \Omega_s=0.01 and below. Further progress in this direction will depend on better understanding of quasar intrinsic variability rather than a mere increase in the volume of data.