  We investigated the effect on the U, B, V magnitudes of the removal of the
emission lines from the spectra of some symbiotic stars and novae during their
nebular phases. We approached this problem by a precise reconstruction of the
composite UV/optical continuum and the line spectrum. Corrections DU, DB, DV
are determined from the ratio of fluxes with and without emission lines. Here
we demonstrate this effect on the case of the symbiotic nova V1016 Cyg during
its nebular phase. We find that about 68, 78 and 66 per-cent of the observed
flux in the U, B and V filters is radiated in the emission lines. The effect
should be taken into account before using the observed color indices of
emission-line objects for diagnosis of their radiation in the continumm.
