  We present a re-analysis of the H$\alpha$ and [OIII] flux data from the only
comprehensive study of the luminosity evolution of nova shells, undertaken
almost two decades ago. We use newly available distances and extinction values,
and include additional luminosity data of 'ancient' nova shells. We compare the
long-term behaviour with respect to nova speed class and light curve type. We
find that, in general, the luminosity as a function of time can be described as
consisting of an initial shallow logarithmic decline or constant behaviour,
followed by a logarithmic main decline phase, with a possible return to a
shallow decline or constancy at very late stages. The luminosity evolution in
the first two phases is likely to be dominated by the expansion of the shell
and the corresponding changes in volume and density, while for the older nova
shells, the interaction with the interstellar medium comes into play. The slope
of the main decline is very similar for almost all groups for a given emission
line, but it is significantly steeper for [OIII], compared to H$\alpha$, which
we attribute to the more efficient cooling provided by the forbidden lines. The
recurrent novae are among the notable exceptions, along with the plateau light
curve type novae and the nova V838 Her. We speculate that this is due to the
presence of denser material, possibly in the form of remnants from previous
nova eruptions, or of planetary nebulae, As a by-product of our study, we
revised the identification of all novae included in our investigation with
sources in the Gaia Data Release 2 catalogue.
