  The performance gain by allowing half-duplex source cooperation is studied
for Gaussian interference channels. The source cooperation is {\em in-band},
meaning that each source can listen to the other source's transmission, but
there is no independent (or orthogonal) channel between the sources. The
half-duplex constraint supposes that at each time instant the sources can
either transmit or listen, but not do both.
  Our main result is a characterization of the sum capacity when the
cooperation is bidirectional and the channel gains are symmetric. With
unidirectional cooperation, we essentially have a cognitive radio channel. By
requiring the primary to achieve a rate close to its link capacity, the best
possible rate for the secondary is characterized within a constant. Novel inner
and outer bounds are derived as part of these characterizations.
