  What are Type II-Linear supernovae (SNe II-L)? This class, which has been ill
defined for decades, now receives significant attention -- both theoretically,
in order to understand what happens to stars in the ~15-25Mo range, and
observationally, with two independent studies suggesting that they cannot be
cleanly separated photometrically from the regular hydrogen-rich SNe II-P
characterised by a marked plateau in their light curve. Here, we analyze the
multi-band light curves and extensive spectroscopic coverage of a sample of 35
SNe II and find that 11 of them could be SNe II-L. The spectra of these SNe are
hydrogen deficient, typically have shallow Halpha absorption, may show indirect
signs of helium via strong OI 7774 absorption, and have faster line velocities
consistent with a thin hydrogen shell. The light curves can be mostly
differentiated from those of the regular, hydrogen-rich SNe II-P by their
steeper decline rates and higher luminosity, and we propose as a defining
photometric characteristic the decline in the V band: SNe II-L seem to decline
by more than 0.5 mag from peak brightness by day 50 after explosion. Using our
sample we provide template light curves for SNe II-L and II-P in 4 photometric
bands.

High Energy Astrophysical Phenomena