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We reconsider Hara's theorem in its relation to the well-known properties ofbeta-decay. All assumptions necessary for the theorem to be true are explicitlyformulated. Further, we study the W-exchange contribution to weak radiativedecays and show that it does not violate Hara's theorem. However, thiscontribution reveals the essential role of particle mixing in symmetryconsiderations and some peculiar features of gauge-invariant amplitudes underperturbative expansion. Together they explain an effect, which was treated ascontradicting Hara's theorem, without any violation. The properties ofW-exchange we describe here may have more general importance and should betaken into account in further detailed calculations of weak processes.Comment: 14 pages, LATEX, no figure

Hara’s theorem and W—exchange in hyperon weak radiative decays