Spectral analyses of supernovae have allowed them to be broadly divided into two classes, type I and type II. Supernova 1993J was identified1-3 as a type II supernova following the detection of a weak hydrogen line in its early spectrum. But the optical light curve4 of SN1993J is atypical for this class of supernova, with the intensity rising to a second maximum after the initial outburst. The light curve around the second maximum more closely resembles that of the type Ib supernova 1983N (ref. 5), the progenitor of which may have been a helium star6. Here we show that the secondary brightening and subsequent decay of the light curve can be explained by the radioactive decay of 56Co; combined with the early spectrum, this suggests that the progenitor may have been a red supergiant with an unusually thin hydrogen-rich envelope. If this model is correct, the spectrum will rapidly evolve from type II to type Ib, in which case SN1993J would be classified as a type lib supernova7-8. We suggest that the progenitor was in a binary system, and had lost most of its hydrogen envelope to the companion star before the explosion.
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