From a sample of 1.5 million Z(0) --> <q(q)over bar> decays collected
by the DELPHI detector during 1991, 1992 and 1993, the production cros
s section for isolated final state photons is measured and is compared
with the O(alpha, alpha(s)) matrix element calculations implemented i
n the EEPRAD and GNJETS Monte Carlo generators. The observed photon yi
eld is used to derive the electroweak couplings of charge 2/3 (u-type)
and charge 1/3 (d-type) quarks to the Z(0) boson. The measured values
C-2/3 = 0.91(-0.36)(+0.25) and c(1/3) = 1.62(-0.17)(+0.24) are compat
ible with the Standard Model prediction, C-2/3 = 1.145 and c(1/3) = 1.
477.