  We present new data for five under-luminous type II-plateau supernovae (SNe
IIP), namely SN 1999gn, SN 2002gd, SN 2003Z, SN 2004eg and SN 2006ov. This new
sample of low-luminosity SNe IIP (LL SNe IIP) is analyzed together with similar
objects studied in the past. All of them show a flat light curve plateau
lasting about 100 days, an under luminous late-time exponential tail, intrinsic
colours that are unusually red, and spectra showing prominent and narrow
P-Cygni lines. A velocity of the ejected material below 10^3 km/s is inferred
from measurements at the end of the plateau. The 56Ni masses ejected in the
explosion are very small (less than 10^-2 solar masses). We investigate the
correlations among 56Ni mass, expansion velocity of the ejecta and absolute
magnitude in the middle of the plateau, confirming the main findings of Hamuy
(2003), according to which events showing brighter plateau and larger expansion
velocities are expected to produce more 56Ni. We propose that these faint
objects represent the low luminosity tail of a continuous distribution in
parameters space of SNe IIP. The physical properties of the progenitors at the
explosion are estimated through the hydrodynamical modeling of the observables
for two representative events of this class, namely SN 2005cs and SN 2008in. We
find that the majority of LL SNe IIP, and quite possibly all, originate in the
core-collapse of intermediate mass stars, in the mass range 10-15 solar masses.

Solar and Stellar Astrophysics