I am actually curious about the spin of excitons. Since an exciton is a bount electron-hole pair, its spin is the sum of the spin of electron and hole, AM I RIGHT? If I use the usual addition of angular momentum in quantum mechanics, I get spin 2,1, and 0. I used the fact that electrons have z-spin=+-1/2, light holes z-spin=+-1/2, and heavy holes z-spin=+-3/2. Is my method correct? However, when I look at publications about excitons, there are only two spins, spin=1 bright excitons, and spin=2 dark excitons What happened to the spin=0? Maybe there's no such thing but what will the spin of an exciton made of z-spin=1/2 electron and a z-spin=-1/2 light hole? z-spin = means projection on z-axis. Thanks!