  A simple exactly solvable model is proposed for describing the decays
$\Upsilon (4S) \rightarrow B^0\bar B^0$ and $\Upsilon (4S) \rightarrow B^+B^-$.
Our predictions agree with available experimental data. Using this model, we
analyze the Coulomb effects in the spectra of these decays. It is shown that
the frequently used assumption of factorization of Coulomb effects is not
fulfilled. The Coulomb interaction leads to the difference in the positions and
heights of the peaks corresponding to the charged and neutral modes.
  As a result, the ratio of probability of $\Upsilon (4S)\rightarrow B^+B^-$
decay and $\Upsilon (4S) \rightarrow B^0\bar B^0$ decay is a nontrivial
function of energy.
