  Using the SANC system we study the one-loop electroweak standard model
predictions, including virtual and real photon emission, for the decays of the
on-shell vector boson, W --> L ANTI-NU (GAMMA). The complete one-loop
corrections and exact photon emission matrix element are taken into account.
For the phase-space integration, the Monte Carlo technique is used. This
provides a useful element, first for the evaluation of the theoretical
uncertainty of PHOTOS. Later we analyse the source of the differences between
SANC and PHOTOS and we calculate the additional weight, which once installed,
improves predictions of PHOTOS simulations. We can conclude that, after the
correction of the weight is implemented, the theoretical uncertainty of PHOTOS
simulations due to an incomplete first-order matrix element is reduced to below
alpha/pi, for observables not tagging the photon in a direct way, and to 10%
otherwise. This is interesting for applications in the phenomenology of the
ongoing LEP2 and future LC and LHC experimental studies.
