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.
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