Circumstellar interaction in type Ibn supernovae and SN 2006jc
  I analyse peculiar properties of light curve and continua of enigmatic Ibn
supernovae, including SN 2006jc, and argue in favour of the early strong
circumstellar interaction. This interaction explains the high luminosity and
fast flux rise of SN 1999cq, while the cool dense shell formed in shocked
ejecta can explain the smooth early continuum of SN 2000er and unusual blue
continuum of SN 2006jc. The dust is shown to condense in the cool dense shell
at about day 50. Monte Carlo modelling of the He I 7065 \AA line profile
affected by the dust occultation supports a picture, in which the dust resides
in the fragmented cool dense shell, whereas He I lines originate from
circumstellar clouds shocked and fragmented in the forward shock wave.
