  We study the complete merger of two massive stars inside a common envelope
and the subsequent evolution of the merger product, a rapidly rotating massive
supergiant. Three qualitatively different types of mergers have been identified
and investigated in detail, and the post-merger evolution has been followed to
the immediate presupernova stage. The ``quiet merger'' case does not lead to
significant changes in composition, and the star remains a red supergiant. In
the case of a ``moderate merger'', the star may become a blue supergiant and
end its evolution as a blue supergiant, depending on the core to total mass
ratio (as may be appropriate for the progenitor of SN 1987A). In the case of
the most effective ``explosive merger'', the merger product stays a red giant.
In last two cases, the He abundance in the envelope is increased drastically,
but significant s-processing is mainly expected in the ``explosive merger''
case.
