The properties of the peculiar type Ia supernova 1991bg -- I. Analysis and discussion of two years of observations

Observations of the peculiar type Ia SN~1991bg in NGC~4374 collected at ESO--La Silla and Asiago are presented and discussed. The broad--band light curves in the early months have a narrower peak and a luminosity decline faster than other SNIa while the R and I light curves do not show the secondary peak. The SN is intrinsically very red (\ₘₐₓ=0.74) and faint (Bₘₐₓ=-16.54). The $uvoir$ bolometric light curve of SN~1991bg is fainter with a steeper decline than that of the normal SNIa (e.g. 1992A). This object enhances the correlation which exists between the peak luminosity of SNIa, the decline rate and the kinetic energy. Although a number of spectral peculiarities are present the spectral evolution retains a general resemblance to that of other SNIa until the latest available observation (day 203). This work suggests that the explosion energy was probably a factor 3 to 5 lower than in normal SNIa.