  A general study of pentaquarks built with four quarks in a L=1 state and an
antiquark in S-wave shows that several of such states are forbidden by a
selection rule, which holds in the limit of flavour symmetry, to decay into a
baryon and a meson final state. We identify the most promising \bar{10}
multiplet for the classification of the Theta^+ and Xi^{--} particles recently
discovered with the prediction of a narrow width for both of them.
