Vanishing Fermi--Gamow-Teller mixing in the positron decay of /sup 58/Co

The asymmetry parameter, A/sub ..beta../, in the positron decay of polarized /sup 58/Co has been measured to be +0.341 +- 0.013. Assuming the standard T-invariant V-A weak interaction theory, the Fermi to Gamow-Teller mixing ratio, y = C/sub V/M/sub F//C/sub A/M/sub GT/, was determined to be -0.005 +- 0.008. The vanishing value of y is consistent with the isospin selection rule for the Fermi matrix element.