Abstract
Using the ZEUS detector at HERA, we have searched for heavy excited states of electrons, neutrinos, and quarks in $e^+p$ collisions at a center-of-mass energy of 300~GeV. With an integrated luminosity of 9.4~pb$^{-1}$, no evidence was found for electroweak production and decay of such states. Limits on the production cross section times branching ratio and on the characteristic couplings, $f/\Lambda$, are derived for masses up to 250~GeV. For the particular choice $f/\Lambda = 1/M_{f^*}$, we exclude at the 95% confidence level excited electrons with mass between 30 and 200~GeV, excited electron neutrinos with mass between 40 and 96~GeV, and quarks excited electroweakly with mass between 40 and 169~GeV.