The C-metric is usually understood as describing two black holes which accelerate in opposite
directions under the action of some conical singularity. Here, we examine all the solutions
of this type which represent accelerating sources and investigate the null limit in which the
accelerations become unbounded. We show that the resulting space-times represent spherical
impulsive gravitational waves generated by snapping or expanding cosmic strings.
This is a pre-print. This is also available at: http://xxx.soton.ac.uk/abs/gr-qc/0006092.