  The polarized longitudinal-transverse structure function $\sigma_{LT^\prime}$
measures the interference between real and imaginary amplitudes in pion
electroproduction and can be used to probe the coupling between resonant and
non-resonant processes. We report new measurements of $\sigma_{LT^\prime}$ in
the $N(1440){1/2}^+$ (Roper) resonance region at $Q^2=0.40$ and 0.65 GeV$^2$
for both the $\pi^0 p$ and $\pi^+ n$ channels. The experiment was performed at
Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using
longitudinally polarized electrons at a beam energy of 1.515 GeV. Complete
angular distributions were obtained and are compared to recent phenomenological
models. The $\sigma_{LT^\prime}(\pi^+ n)$ channel shows a large sensitivity to
the Roper resonance multipoles $M_{1-}$ and $S_{1-}$ and provides new
constraints on models of resonance formation.
