  A theory of a pseudogap phase of high-temperature superconductors where
current carriers are translation invariant bipolarons is developed. A
temperature T* of a transition from a pseudogap phase to a normal one is
calculated. For the temperature of a transition to the pseudogap phase, the
isotope coefficient is found. It is shown that the results obtained, in
particular, the possibility of negative values of the isotope coefficient are
consistent with the experiment. New experiments on the influence of the
magnetic field on the isotope coefficient are proposed.
