  Type Ibn supernovae (SNe) are a small yet intriguing class of explosions
whose spectra are characterized by low-velocity helium emission lines with
little to no evidence for hydrogen. The prevailing theory has been that these
are the core-collapse explosions of very massive stars embedded in helium-rich
circumstellar material (CSM). We report optical observations of six new SNe
Ibn: PTF11rfh, PTF12ldy, iPTF14aki, iPTF15ul, SN 2015G, and iPTF15akq. This
brings the sample size of such objects in the literature to 22. We also report
new data, including a near-infrared spectrum, on the Type Ibn SN 2015U. In
order to characterize the class as a whole, we analyze the photometric and
spectroscopic properties of the full Type Ibn sample. We find that, despite the
expectation that CSM interaction would generate a heterogeneous set of light
curves, as seen in SNe IIn, most Type Ibn light curves are quite similar in
shape, declining at rates around 0.1 mag/day during the first month after
maximum light, with a few significant exceptions. Early spectra of SNe Ibn come
in at least two varieties, one that shows narrow P Cygni lines and another
dominated by broader emission lines, both around maximum light, which may be an
indication of differences in the state of the progenitor system at the time of
explosion. Alternatively, the spectral diversity could arise from viewing-angle
effects or merely from a lack of early spectroscopic coverage. Together, the
relative light curve homogeneity and narrow spectral features suggest that the
CSM consists of a spatially confined shell of helium surrounded by a less dense
extended wind.
