Massive stars exploding in a He-rich circumstellar medium – II. The transitional case of SN 2005la

We present photometric and spectroscopic data of the peculiar SN 20051a, an object which shows an optical light curve with some luminosity fluctuations and spectra with comparably strong narrow hydrogen and helium lines, probably of circumstellar nature. The increasing full width at half-maximum velocity of these lines is indicative of an acceleration of the circumstellar material. SN 20051a exhibits hybrid properties, sharing some similarities with both Type IIn supernovae and 2006jc-like (Type Ibn) events. We propose that the progenitor of SN 20051a was a very young Wolf-Rayet (WN-type) star which experienced mass ejection episodes shortly before core collapse.