Abstract
A search for first-generation leptoquarks was performed in electron-proton and positron-proton collisions recorded with the ZEUS detector at HERA in 2003--2007 using an integrated luminosity of $366\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$. Final states with an electron and jets or with missing transverse momentum and jets were analyzed, searching for resonances or other deviations from the standard model predictions. No evidence for any leptoquark signal was found. The data were combined with data previously taken at HERA, resulting in a total integrated luminosity of $498\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$. Limits on the Yukawa coupling, $\ensuremath{\lambda}$, of leptoquarks were set as a function of the leptoquark mass for different leptoquark types within the Buchm\"uller-R\"uckl-Wyler model. Leptoquarks with a coupling $\ensuremath{\lambda}=0.3$ are excluded for masses up to 699 GeV.