  We report the results of our follow-up campaign of the peculiar supernova
ASASSN-15no, based on optical data covering ~300 days of its evolution.
Initially the spectra show a pure blackbody continuum. After few days, the HeI
5876 A transition appears with a P-Cygni profile and an expansion velocity of
about 8700 km/s. Fifty days after maximum, the spectrum shows signs typically
seen in interacting supernovae. A broad (FWHM~8000 km/s) Halpha becomes more
prominent with time until ~150 days after maximum and quickly declines later
on. At these phases Halpha starts to show an intermediate component, which
together with the blue pseudo-continuum are clues that the ejecta begin to
interact with the CSM. The spectra at the latest phases look very similar to
the nebular spectra of stripped-envelope SNe. The early part (the first 40 days
after maximum) of the bolometric curve, which peaks at a luminosity
intermediate between normal and superluminous supernovae, is well reproduced by
a model in which the energy budget is essentially coming from ejecta
recombination and 56Ni decay. From the model we infer a mass of the ejecta Mej
= 2.6 Msun; an initial radius of the photosphere R0 = 2.1 x 10^14 cm; and an
explosion energy Eexpl = 0.8 x 10^51 erg. A possible scenario involves a
massive and extended H-poor shell lost by the progenitor star a few years
before explosion. The shell is hit, heated and accelerated by the supernova
ejecta. The accelerated shell+ejecta rapidly dilutes, unveiling the unperturbed
supernova spectrum below. The outer ejecta start to interact with a H-poor
external CSM lost by the progenitor system about 9 -- 90 years before the
explosion.
