An Emerging Class of Bright, Fast-evolving Supernovae with Low-mass
  Ejecta
  A recent analysis of supernova (SN) 2002bj revealed that it was an apparently
unique type Ib SN. It showed a high peak luminosity, with absolute magnitude
M_R -18.5, but an extremely fast-evolving light curve. It had a rise time of <7
days followed by a decline of 0.25 mag per day in B-band, and showed evidence
for very low mass of ejecta (<0.15 M_Sun). Here we discuss two additional
historical events, SN 1885A and SN 1939B, showing similarly fast light curves
and low ejected masses. We discuss the low mass of ejecta inferred from our
analysis of the SN 1885A remnant in M31, and present for the first time the
spectrum of SN 1939B. The old environments of both SN 1885A (in the bulge of
M31) and SN 1939B (in an elliptical galaxy with no traces of star formation
activity), strongly support old white dwarf progenitors for these SNe. We find
no clear evidence for helium in the spectrum of SN 1939B, as might be expected
from a helium-shell detonation on a white dwarf, suggested to be the origin of
SN 2002bj. Finally, the discovery of all the observed fast-evolving SNe in
nearby galaxies suggests that the rate of these peculiar SNe is at least 1-2 %
of all SNe.
