Extensive optical and near‐infrared (NIR) observations of the Type IIb supernova (SN IIb) 2008ax are presented, covering the first year after the explosion. The light curve is mostly similar in shape to that of the prototypical SN IIb 1993J, but shows a slightly faster decline rate at late phases and lacks the prominent narrow early‐time peak of SN 1993J. From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.07–0.15 M⊙ of 56Ni was produced during the explosion and that the total ejecta mass was between 2 and 5 M⊙, with a kinetic energy of at least 1051 erg. The spectral evolution of SN 2008ax is similar to that of SN Ib/IIb 2007Y, exhibiting high‐velocity Ca ii features at early phases and signs of ejecta–wind interaction from Hα observations at late times. NIR spectra show strong He i lines similar to SN Ib 1999ex and a large number of emission features at late times. Particularly interesting are the strong, double‐peaked He i lines in late NIR spectra, which – together with the double‐peaked [O i] emission in late optical spectra – provide clues for the asymmetry and large‐scale Ni mixing in the ejecta.

The velocity of the inner ejecta of stripped‐envelope core‐collapse supernovae (CC‐SNe) is studied by means of an analysis of their nebular spectra. Stripped‐envelope CC‐SNe are the result of the explosion of bare cores of massive stars (≥8 M⊙), and their late‐time spectra are typically dominated by a strong [O i]λλ6300, 6363 emission line produced by the innermost, slow‐moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped‐envelope CC‐SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the linewidth as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s−1. Observations (<100 d) of stripped‐envelope CC‐SNe have revealed a subclass of very energetic SNe, termed broad‐lined SNe (BL‐SNe) or hypernovae, which are characterized by broad absorption lines in the early‐time spectra, indicative of outer ejecta moving at very high velocity (v≥ 0.1c). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC‐SNe. This might indicate asphericity of the inner ejecta of BL‐SNe, a possibility we investigate using synthetic three‐dimensional nebular spectra.

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