On the exceptional locus of the birational projections of normal surface singularity into a plane

Given a normal surface singularity $(X,Q)$ and a birational morphism to a non- singular surface $\pi :X\to S$, we investigate the local geometry of the exceptional divisor $L$ of $\pi$. We prove that the dimension of the tangent space to $L$ at $Q$ equals the number of exceptional components meeting at $Q$. Consequences relative to the existence of such birational projections contracting a prescribed number of irreducible curves are deduced. A new characterization of minimal singularities is obtained in these terms.