  The peculiar type Ia supernova SN 1997br in ESO 576-G40 was extensively
observed at Beijing Astronomical Observatory and Lick Observatory. In this
paper, we present and discuss the BVRI photometry and the spectra collected
over 3 months, beginning 9 days before maximum brightness. The light curves of
SN 1997br are similar to those of SN 1991T, with slow decline rates after the B
maximum. Well-sampled data before the B maximum show unambiguously that SN
1997br rises more slowly and has a wider peak than normal type Ia supernovae.
The optical color evolution of SN 1997br is also similar to that of SN 1991T.
We estimate the extinction of SN 1997br to be E(B-V) = 0.35+/-0.10 mag by
comparing its BVRI light curves with those of SN 1991T and by measureing the
equivalent width of interstellar Na I D absorption lines. We have conducted a
thorough comparison of the spectroscopic evolution of SN 1997br, SN 1991T, and
SN 1994D. Although SN 1997br is generally very similar to SN 1991T, it shows
some interesting differences at various epoches. Spectra of SN 1997br seem to
indicate an earlier transition to the dominant phase of Fe-peak elements after
the B maximum. Si II lines in SN 1997br show a very short duration after the B
maximum. We discuss the implications of our observations of SN 1997br for
models of type Ia supernovae. Specifically, we suggest that some SNe Ia may
result from decelerated detonations of white dwarfs.
