  We present optical observations of the peculiar Type Ibn supernova (SN Ibn)
OGLE-2012-SN-006, discovered and monitored by the OGLE-IV survey, and
spectroscopically followed by PESSTO at late phases. Stringent pre-discovery
limits constrain the explosion epoch with fair precision to JD = 2456203.8 +-
4.0. The rise time to the I-band light curve maximum is about two weeks. The
object reaches the peak absolute magnitude M(I) = -19.65 +- 0.19 on JD =
2456218.1 +- 1.8. After maximum, the light curve declines for about 25 days
with a rate of 4 mag per 100d. The symmetric I-band peak resembles that of
canonical Type Ib/c supernovae (SNe), whereas SNe Ibn usually exhibit
asymmetric and narrower early-time light curves. Since 25 days past maximum,
the light curve flattens with a decline rate slower than that of the 56Co to
56Fe decay, although at very late phases it steepens to approach that rate. An
early-time spectrum is dominated by a blue continuum, with only a marginal
evidence for the presence of He I lines marking this SN Type. This spectrum
shows broad absorptions bluewards than 5000A, likely O II lines, which are
similar to spectral features observed in super-luminous SNe at early epochs.
The object has been spectroscopically monitored by PESSTO from 90 to 180 days
after peak, and these spectra show the typical features observed in a number of
SN 2006jc-like events, including a blue spectral energy distribution and
prominent and narrow (v(FWHM) ~ 1900 km/s) He I emission lines. This suggests
that the ejecta are interacting with He-rich circumstellar material. The
detection of broad (10000 km/s) O I and Ca II features likely produced in the
SN ejecta (including the [O I] 6300A,6364A doublet in the latest spectra) lends
support to the interpretation of OGLE-2012-SN-006 as a core-collapse event.
