  We obtain new intersecting 5-brane, string and pp-wave solutions in the
heterotic string on a torus and on a K3 manifold. In the former case the
5-brane is supported by Yang-Mills instantons, and in the latter case both the
5-brane and the string are supported by the instantons. The instanton moduli
are parameterised by the sizes and locations of the instantons. We exhibit two
kinds of phase transition in which, for suitable choices of the instanton
moduli, a 5-brane and/or a string can be created. One kind of phase transition
occurs when the size of an instanton vanishes, while the other occurs when a
pair of Yang-Mills instantons coalesce. We also study the associated
five-dimensional black holes and the implications of these phase transitions
for the black-hole entropy. Specifically, we find that the entropy of the
three-charge black holes is zero when the instantons are separated and of
non-zero scale size, but becomes non-zero (which can be counted miscrospically)
after either of the phase transitions.
