  The NA62 experiment at CERN aims to use rare kaon decays to search for new
phenomena beyond the Standard Model. During the current short term phase (data
taking completed in years 2007-8), the ratio $R_K = \Gamma(K^+ \rightarrow e
\nu_e (\gamma)) / \Gamma(K^+ \rightarrow \mu \nu_{\mu} (\gamma))$ of leptonic
decay rates is studied, which tests the structure of weak interactions and
lepton flavour universality. In this paper, the $R_K$ analysis is summarized,
and the preliminary result is discussed, based on 59963 $K^+ \rightarrow e
\nu_e $ candidates collected in 2007.
