  Consider a closed oriented four-manifold $X$ that contains an embedded sphere
of self-intersection number $(-1)$. Suppose that $b_2^+(X) \leq 3$. A
Riemannian metric on $X$ is found such that the cohomology class dual to this
sphere is represented by an anti-self-dual harmonic form. Moreover, such a
metric exists even for multiple disjoint embedded $(-1)$-spheres.
