  The D0-D6 system, which is not supersymmetric in the absence of a
Neveu-Schwarz B-field, becomes supersymmetric if a suitable constant B-field is
turned on. On one side of the supersymmetric locus, this system has a BPS bound
state, and on the other side it does not. After compactification on T^6, this
gives a simple example in which the number of 1/8 BPS states jumps as the
moduli of the compactification are changed. The D0-D8 system in a B-field has
two different supersymmetric loci, only one of which is continuously connected
to the familiar supersymmetric D0-D8 system without a B-field. In a certain
range, the D0-D8 system also has a BPS bound state. In the limit in which the
B-field goes to infinity, supersymmetric D0-D6 and D0-D8 systems and their
bound states can be studied using noncommutative Yang-Mills theory.
