Observations of the type II SN 1996L reveal the presence of a slowly
expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow
emission features are visible in the early spectra superposed on the normal SN
spectrum. Within about two months these features develop narrow symmetric
P-Cygni profiles. About 100 days after the explosion the light curve suddenly
flattens, the spectral lines broaden and the Halpha flux becomes larger than
what is expected from a purely radioactive model. These events are interpreted
as signatures of the onset of the interaction between the fast moving ejecta
and a slowly moving outer shell of matter ejected before the SN explosion. At
about 300 days the narrow lines disappear and the flux drops until the SN fades
away, suggesting that the interaction phase is over and that the shell has been
swept away. Simple calculations show that the superwind episode started 9 yr
before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3)
M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden
lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day
270 show relatively strong emission lines of HeI. These lines are narrower than
other emission lines coming from the SN ejecta, but broader than those from the
CSM. These high excitation lines are probably the result of non-thermal
excitation and ionization caused by the deposition of the gamma-rays emitted in
the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.