The OGLE-II event sc5_2859 : a Classical Nova outburst?

The OGLE-II event sc5_2859 was previously identified as the third longest microlensing event ever observed. Additional photometric observations from the EROS (Experience de Recherche d'Objets Sombres) survey and spectroscopic observations of the candidate star are used to test the microlensing hypothesis.The combined OGLE and EROS data provide a high quality coverage of the light curve. The colour of the sc5_2859 event is seen to change with time. A spectrum taken in 2003 exhibits a strong Halpha emission line. The additionnal data show that the OGLE-II sc5_2859 event is actually a classical nova outburst.