The type Ia supernova 1997br in ESO 576-G40

The peculiar type Ia supernova SN 1997br in ESO 576-G40 was extensively obs erved at Beijing Astronomical Observatory and Lick Observatory. In this pap er, we present and discuss the BVRI photometry and the spectra collected ov er 3 months, beginning 9 days before maximum brightness. The light curves o f 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 t hat SN 1997br rises more slowly and has a wider peak than normal type Ia su pernovae. 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 b y measuring the equivalent width of interstellar Na I D absorption lines. W e have conducted a thorough comparison of the spectroscopic evolution of SN 1997br, SN 1991T., and SN 1994D. Although SN 1997br is generally very simi lar to SN 1991T, it shows some interesting differences at various epochs. S pectra of SN 1997br seem to indicate an earlier transition to the dominant phase of Fe-peak elements after the B maximum. Si rr lines in SN 1997br sho w a very short duration after the B maximum. We discuss the implications of our observations of SN 1997br for models of type Ia supernovae. Specifical ly, we suggest that some SNe Ia may result from decelerated detonations of white dwarfs.