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 i. Light curve modelling suggests that the progenitor probably had a very large radius (∼1014 cm) and that a considerable amount of 56Ni was synthesized during the explosion (∼0.7 M⊙). We suggest a progenitor model of SN 1991D that involves a binary system.