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Extensive optical and near‐infrared observations of the Type Ib supernova (SNIb) 1999dn are presented, covering the first year after explosion. These new data turn this object, already considered a prototypical SNIb, into one of the best observed objects of its class. The light curve of SN 1999dn is mostly similar in shape to that of other SNeIb but with a moderately faint peak  ( M V  =−17.2 mag) . From the bolometric light curve and ejecta expansion velocities, we estimate that about  0.11 M ⊙   of   56  Ni were produced during the explosion and that the total ejecta mass was  4–6 M ⊙   with a kinetic energy of at least  5 × 10 51   erg. The spectra of SN 1999dn at various epochs are similar to those of other stripped envelope SNe showing clear presence of H at early epochs. The high explosion energy and ejected mass, along with the small flux ratio [Ca  ii ]/[O  i ] measured in the nebular spectrum, together with the lack of signatures of dust formation and the moderate metallicity environment is not inconsistent with a single massive progenitor  ( M  ZAMS ≥ 23–25 M ⊙ )  for SN 1999dn.

The Type Ib SN 1999dn: one year of photometric and spectroscopic monitoring ★