  We investigate the effect on the U, B, V, Rc and Rj magnitudes of the removal
of emission lines from a spectrum. We determined "Dm" corrections from the
ratio of fluxes with and without emission lines, transmitted from the object
through the photometric filter. An exact and a simplified approach for
operative use were applied. The effect was demonstrated for classical symbiotic
stars, symbiotic novae and the classical nova V1974 Cyg. It was found that
about 20-30, 30-40, 10 and 26/20 percents of the observed flux in the U, B, V
and Rc/Rj filter, respectively, is radiated in the emission lines of the
investigated classical symbiotic stars. The largest effect was found for
symbiotic novae (RR Tel and V1016 Cyg) and the classical nova V1974 Cyg at 210
days (in average of 74, 79, 56 and 66/60 percents). Examples for practical
application are suggested.
