String and five-brane solitons: Singular or non-singular

We ask whether the recently discovered superstring and superfive-brane solution of D=10 supergravity admit the interpretation of non-singular solitons even though, in the absence of Yang-Mills fields, they exhibit curvature singularities at the origin. We answer the question using a test probe/source approach, and find that the nature of the singularity is probe dependent. If the test probe and source are both superstrings or both super five-branes, one falls into the other in a finite proper time and the singularity is real, whereas if one is a superstring and the other a super five-brane it takes an infinite proper time (the force is repulsive !) and the singularity is harmless. Black strings and five-branes, on the other hand, always display real singularities. (orig.)