  For a semisimple factorizable Hopf algebra over a field of characteristic
zero, we show that the value that an integral takes on the inverse Drinfel'd
element differs from the value that it takes on the Drinfel'd element itself at
most by a fourth root of unity. This can be reformulated by saying that the
central charge of the Hopf algebra is an integer. If the dimension of the Hopf
algebra is odd, we show that these two values differ at most by a sign, which
can be reformulated by saying that the central charge is even. We give a
precise condition on the dimension that determines whether the plus sign or the
minus sign occurs. To formulate our results, we use the language of modular
data.
