  The recent interest in a light gauge boson in the framework of an extra U(1)
symmetry motivates searches in the mass range below 1 GeV.
  We present a search for such a particle, the dark photon, in ${\rm e^+
e^-}\rightarrow {\rm U}\gamma$, U$\rightarrow \pi^+\pi^-$ based on 28 million
$\mathrm{e^+ e^-} \rightarrow \pi^+ \pi^-\gamma$ events collected at DA$\Phi$NE
by the KLOE experiment. The $\pi^+ \pi^-$ production by initial-state radiation
compensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow
\mathrm{e^+ e^-}$, $\mu^+\mu^-$ searches due to the small branching ratios in
the $\rho-\omega$ resonance region. We found no evidence for a signal and set a
limit at 90\% CL on the mixing strength between the photon and the dark photon,
$\varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV
this new limit is more stringent than previous ones.
