The tau-->nu tauetapi process in and beyond QCD

We estimate various sources of the tau-->nu tauetapi process by using QCD-duality sum rule information on the decay constants of standard and exotic mesons. We use the pi--> lnu l decay in order to constrain the effects of new interactions. We conclude from our analysis that in al cases, the tau-->nu tauetapi branching ration is small. We also point out that the present data on the tau-->nupi process already provide an upper limit of 0.1% on the etapi mode while the tau-->nu tauKK¯ data imply an upper limit of 0.5% if the a 0 (980)- meson is a four-quark state.