  The notion of a coalgebra-Galois extension is defined as a natural
generalisation of a Hopf-Galois extension. It is shown that any
coalgebra-Galois extension induces a unique entwining map $\psi$ compatible
with the right coaction. For the dual notion of an algebra-Galois coextension
it is also proven that there always exists a unique entwining structure
compatible with the right action.
