In this paper we present spectroscopic and photometric observations for four core collapse supernovae ( SNe ) , namely SNe 1994N , 1999br , 1999eu and 2001dc .
Together with SN 1997D , we show that they form a group of exceptionally low–luminosity events .
These SNe have narrow spectral lines ( indicating low expansion velocities ) and low luminosities at every phase ( significantly lower than those of typical core–collapse supernovae ) .
The very low luminosity during the ^ { 56 } Co radioactive decay tail indicates that the mass of ^ { 56 } Ni ejected during the explosion is much smaller ( M _ { Ni } \approx 2–8 \times 10 ^ { -3 } M _ { \odot } ) than the average ( M _ { Ni } \approx 6–10 \times 10 ^ { -2 } M _ { \odot } ) .
Two supernovae of this group ( SN 1999br and SN 2001dc ) were discovered very close to the explosion epoch , allowing us to determine the lengths of their plateaux ( \approx 100 days ) as well as establishing the explosion epochs of the other , less–completely observed SNe .
It is likely that this group of SNe represent the extreme low–luminosity tail of a single continuous distribution of SN II–P events .
Their kinetic energy is also exceptionally low .
Although an origin from low mass progenitors has also been proposed for low–luminosity core–collapse SNe , recent work provides evidence in favour of the high mass progenitor scenario .
The incidence of these low–luminosity SNe could be as high as 4–5 \% of all type II SNe .