  (Abridged) We studied the brightness and spectral evolution of the young
eruptive star V1647 Ori during its recent outburst in the period 2004 February
- 2006 Sep. We performed a photometric follow-up in the bands V, R_C, I_C, J,
H, K_s as well as visible and near-IR spectroscopy. The main results are as
follows: The brightness of V1647 Ori stayed more than 4 mag above the
pre-outburst level until 2005 October when it started a rapid fading. In the
high state we found a periodic component in the optical light curves with a
period of 56 days. The delay between variations of the star and variations in
the brightness of clump of nearby nebulosity corresponds to an angle of 61+/-14
degrees between the axis of the nebula and the line of sight. A steady decrease
of HI emission line fluxes could be observed. In 2006 May, in the quiescent
phase, the HeI 1.083 line was observed in emission, contrary to its deep
blueshifted absorption observed during the outburst. The J-H and H-K_s color
maps of the infrared nebula reveal an envelope around the star. The color
distribution of the infrared nebula suggests reddening of the scattered light
inside a thick circumstellar disk. We show that the observed properties of
V1647 Ori could be interpreted in the framework of the thermal instability
models of Bell et al. (1995). V1647 Ori might belong to a new class of young
eruptive stars, defined by relatively short timescales, recurrent outbursts,
modest increase in bolometric luminosity and accretion rate, and an
evolutionary state earlier than that of typical EXors.
