  A theory of BEC interferometry in an unsymmetrical double-well trap has been
developed for small boson numbers, based on the two-mode approximation. The
bosons are initially in the lowest mode of a single well trap, which is split
into a double well and then recombined. Possible fragmentations into separate
BEC states in each well during the splitting/recombination process are allowed
for. The BEC is treated as a giant spin system, the fragmented states are
eigenstates of S^2 and Sz. Self-consistent sets of equations for the amplitudes
of the fragmented states and for the two single boson mode functions are
obtained. The latter are coupled Gross-Pitaevskii equations. Interferometric
effects may be measured via boson numbers in the first excited mode
