  We present a simple sandwich gravitational wave of the Robinson-Trautman
family. This is interpreted as representing a shock wave with a spherical
wavefront which propagates into a Minkowski background minus a wedge. (i.e. the
background contains a cosmic string.) The deficit angle (the tension) of the
string decreases through the gravitational wave, which then ceases. This leaves
an expanding spherical region of Minkowski space behind it. The decay of the
cosmic string over a finite interval of retarded time may be considered to
generate the gravitational wave.
