Threshold kaon photo- and electroproduction in SU(3) baryon chiral perturbation theory

We calculate the processes $\gamma p \to K^0 \Sigma^+ , K^+ \Sigma^0, K^+ \Lambda$ in three flavor heavy baryon chiral perturbation theory to one loop. Some low--energy constants are fixed from single baryon properties while others are estimated by means of resonance saturation. The total cross sections are comparable with the few existing data points in the threshold region. The angular dependences of the recoil polarization of the $\Lambda$ and $\Sigma^0$ show most features of the ones measured at ELSA. We also predict the isovector charge radius of the $\Sigma^+$, $< r^2_V >_{\Sigma^+} = (0.30 \pm 0.03) fm^2$.