The existence of a dilaton ( or moduli ) with gravitational-strength coupling to matter imposes stringent constraints on the allowed energy scale of cosmic strings , \eta .
In particular , superheavy gauge strings with \eta \sim 10 ^ { 16 } { GeV } are ruled out unless the dilaton mass m _ { \phi } \mathrel { \hbox to 0.0 pt { \raise 2.1973 pt \hbox { $ > $ } } { \lower 2.1973 pt% \hbox { $ \sim$ } } } 100 { TeV } , while the currently popular value m _ { \phi } \sim 1 { TeV } imposes the bound \eta \mathrel { \hbox to 0.0 pt { \raise 2.1973 pt \hbox { $ < $ } } { \lower 2.1973 pt \hbox { $% \sim$ } } } 3 \times 10 ^ { 11 } { GeV } .
Similar constraints are obtained for global topological defects .
Some non-standard cosmological scenarios which can avoid these constraints are pointed out .