The simplest extension of Starobinsky inflation
  We consider the simplest extension to the Starobinsky model, by allowing an
extra scalar field to help drive inflation. We perform our analysis in the
Einstein frame and calculate the power spectra at the end of inflation to
second order in the slow--roll parameters. We find that the model gives
predictions in great agreement with the current Planck data without the need
for fine-tuning. Our results encourage current efforts to embed the model in a
supergravity setting.
