  Hubble Space Telescope images of two early-type galaxies harboring both
nuclear and outer stellar discs are studied in detail. By means of a
photometric decomposition, the images of NGC 4342 and NGC 4570 are analyzed and
the photometric properties of the nuclear discs investigated. We find a
continuity of properties in the parameter space defined by the central surface
brightness and the scalelength of discs in spirals, S0s and embedded discs in
ellipticals, in the sense that the nuclear discs extend the observed disc
properties even further towards smaller scalelengths and brighter central
surface brightnesses. When including the nuclear discs, disc properties span
more than four orders of magnitude in both scalelength and central surface
brightness. The nuclear discs studied here are the smallest and brightest
stellar discs known, and as such, they are as extreme in their photometric
properties as Malin I, when compared to typical galactic discs that obey
Freeman's law. We discuss a possible formation scenario in which the
double-disc structure observed in these galaxies has been shaped by now
dissolved bars. Based on the fact that the black holes known to exist in some
of these galaxies have masses comparable to those of the nuclear discs, we
explore a possible link between the black holes and the nuclear discs.
