  We study the merging of massive stars inside a common envelope for binary
systems consisting of a red supergiant with a mass of 15-20 Msun and a
main-sequence companion of 1-5 Msun. We are particularly interested in the
stage when the secondary, having overfilled its Roche lobe inside the common
envelope, starts to transfer mass to the core of the primary at a very high
mass-transfer rate and the subsequent nucleo-synthesis in the core-impact
region. Using a parametrized model for the structure of the envelope at this
stage, we perform 2-dimensional hydrodynamical calculations with the Munich
Prometheus code to calculate the dynamics of the stream emanating from the
secondary and its impact on the core of the primary. We find that, for the
lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich
material can penetrate deep into the primary core where nucleosynthesis through
the hot CNO cycle can take place and that the associated neutron exposure may
be sufficiently high for significant s-processing. For mass-transfer rates at
the high end of our estimated range and higher densities in the stream, the
stream impact can lead to the dredge-up of helium, but the neutron production
is too low for significant s-processing.
