  Pre-outburst signal (a decrease of the optical brightness) just before the
outburst is clearly detected in the observations of the T CrB obtained before
and during the 1946 outburst. A similar decrease is also visible in the light
curve of RS Oph during the 2021 outburst. We suppose that this is due to
formation of a thick, dense envelope around the white dwarf, and we estimate
its size (1000 - 2000 km), mass (5.10$^{-8}$ - 6.10$^{-7}$ M$_\odot$) and
average density (5 - 16 g cm$^{-3}$).
