The exceptionally bright type Ib supernova 1991D

Photometric and spectroscopic observations of the peculiar Type Ib supernova 1991D are presented. SN 1991D was exceptionally bright for a Type Ib supernova. The He I lines were rather weak and the velocity at the photosphere as a function of time was unusually low. Comparison of the observed and synthetic spectra indicates that either hydrogen was ejected with a minimum velocity of 12 000 km s(-1) or the spectrum contained features caused by lines of Ne </x>i<x>. Light curve modelling suggests that the progenitor probably had a very large radius (similar to10(14) cm) and that a considerable amount of Ni-56 was synthesized during the explosion (similar to0.7 M.). We suggest a progenitor model of SN 1991D that involves a binary system.