  The evolution of the luminosity of nova shells in the century following the
nova outburst is studied for the lines H-alpha, H-beta, and [O III]. About 1200
flux measurements from 96 objects have been collected from the literature, from
unpublished observations, from the HST archive, or from new narrow-band filter
imaging. For most objects, the distance and reddening is known (or newly
determined), and luminosities were calculated from the observed fluxes. The
luminosity data were combined in five groups, according to nova light curve
type (very fast, fast, moderately fast, slow, recurrent); some objects were
re-assigned to other groups for a better fit of the luminosity data to the
general trend. For very fast, fast and moderately fast novae, the slope of the
[O III] decline is very similar, leading to a basic `switchoff' of [O III]
emission after 11, 23 and 24 years, respectively. For the same speed classes,
the slope of the Balmer luminosity is quite similar. In contrast to all types
of fast novae, the decline in Balmer luminosity is more rapid in slow novae.
However, the slope in [O III] is more gentle; slow novae still show [O III]
emission after 100 years. Thus shells of slow novae are still hot after one
century; the same applies for the shells of the very fast nova GK Per and the
recurrent nova T Pyx, which interact with circumstellar material.In recurrent
novae, [O III] is usually inconspicuous or absent. In objects with giant
companions, the Balmer luminosity decreases very slowly after an outburst,
which may be an effect of line blending of material from the ejecta and the
giant wind. On the other hand, objects with dwarf companions show a very rapid
decline in Balmer luminosity.
