The spinorial \tau-invariant and 0-dimensional surgery

Let $M$ be a compact manifold with a metric $g$ and with a fixed spin structure $\chi$. Let $\lambda\_1^+(g)$ be the first non-negative eigenvalue of the Dirac operator on $(M,g,\chi)$. We set $$\tau(M,\chi):= \sup \inf \lambda\_1^+(g)$$ where the infimum runs over all metrics $g$ of volume 1 in a conformal class $[g\_0]$ on $M$ and where the supremum runs over all conformal classes $[g\_0]$ on $M$. Let (M^#,\chi^#) be obtained from $(M,\chi)$ by 0-dimensional surgery. We prove that \tau(M^#,\chi^#)\geq \tau(M,\chi). As a corollary we can calculate $\tau(M,\chi)$ for any Riemann surface $M$.