eta -> pi(+) pi(-) pi (0)gamma in chiral perturbation theory

We analyse the radiative decay eta --> pi(+)pi(-)pi(0)gamma in the low-ener gy expansion of the Standard Model. We employ the notion of "generalized br emsstrahlung" to take full advantage of the theoretical and experimental in formation on the corresponding non-radiative eta --> 3 pi decay. The direct emission amplitude of O(p(4)) is due to one-loop diagrams with intermediat e pions (isospin violating) and kaons (isospin conserving). The isospin con serving contributions to direct emission, including Vector meson exchange a ppearing at O(p(6)), are suppressed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.