  V838 Mon is marking one of the most mysterious stellar outbursts on record.
The spectral energy distribution of the progenitor resembles an under-luminous
F main sequence star (at V=15.6 mag), that erupted into a cool supergiant
following a complex and multi-maxima lightcurve (peaking at V=6.7 mag). The
outburst spectrum show BaII, LiI and lines of several s-elements, with wide
P-Cyg profiles and a moderate and retracing emission in the Balmer lines. A
light-echo discovered expanding around the object helped to constrain the
distance (d=790+/-30 pc), providing M_V=+4.45 in quiescence and M_V=-4.35 at
optical maximum (somewhat dependent on the still uncertain E(B-V)=0.5
reddening). The general outburst trend is toward lower temperatures and larger
luminosities, and continuing so at the time of writing. The object properties
conflict with a classification within already existing categories: the
progenitor was not on a post-AGB track and thus the similarities with the
born-again AGB stars FG Sge, V605 Aql and Sakurai's object are limited to the
cool giant spectrum at maximum; the cool spectrum, the moderate wind velocity
(500 km/sec and progressively reducing) and the monotonic decreasing of the low
ionization condition argues against a classical nova scenario. The closest
similarity is with a star that erupted into an M-type supergiant discovered in
M31 by Rich et al. (1989), that became however much brighter by peaking at
M_V=-9.95, and with V4332 Sgr that too erupted into an M-type giant (Martini et
al. 1999) and that attained a lower luminosity, closer to that of V838 Mon.
M31-RedVar, V4332 Sgr and V838 Mon could be manifestations of the same and new
class of astronomical objects.
