  Evolution in the spectrum of very slow novae PU Vul and V723 Cas during their
transition from the optical maximum to the nebular phase is investigated using
the method of disentangling the composite UV/optical spectra. Model SEDs
suggested that a transient decrease in the WD luminosity, during the decline
from the maximum, was caused by a negative beaming effect, when a neutral disk
around the WD was formed. When the disk disappeared, the luminosity increased
again to values from the beginning of the outburst (in the case of V723 Cas,
at/above the Eddington limit). This suggests the presence of a mechanism
maintaining a high energy output for a much longer time than it is predicted by
the current theories. Similarity of light curves, but enormous difference of
the separation between the components of PU Vul and V723 Cas binaries suggest
that the mechanism is basically powered by the accretor.
