  We use rudiments of the Seiberg-Witten gluing theory for trivial circle
bundles over a Riemann surface to relate de Seiberg-Witten basic classes of two
$4$-manifolds containing Riemann surfaces of the same genus and
self-intersection zero with those of the $4$-manifold resulting as a connected
sum along the surface. We study examples in which this is enough to describe
completely the basic classes.
