  A search for the leptonic decays of W bosons produced in the reaction e^+
p\to e^+ W^\pm X at a centre-of-mass energy of 300 GeV has been performed with
the ZEUS detector at HERA using an integrated luminosity of 47.7 pb^-1 . Three
events consistent with W\to e\nu decay are found, giving a cross section of 0.9
+1.0 -0.7 \pm 0.2 pb, in good agreement with the Standard Model prediction. The
corresponding 95% C.L. upper limit on the cross section is 3.3 pb. A search for
the decay W\to \mu\nu has a smaller selection efficiency and yields no
candidate events. Limits on the cross section for W production with large
hadronic transverse momentum have been obtained. A search for
high-transverse-momentum isolated tracks in events with large missing
transverse momentum yields results in good agreement with Standard Model
expectations, in contrast to a recent report by the H1 collaboration of the
observation of an excess of such events.
