OO Serpentis is a deeply embedded young star which brightened by
5 mag in the K band between 1994 and 1995, and gradually faded
afterwards. In many respects the star and its outburst is very
similar to the well-known young eruptive stars (FUors and EXors).
These outbursts are closely connected to the circumstellar
material, thus infrared observations are essential in
understanding the physics of these objects. Since very few
outbursts have ever been documented at infrared wavelengths, we
monitored OO Ser for 20 months with the Infrared Space
Observatory in the 3.6 100
m wavelength range. We
complemented this dataset with new (ground-based and spaceborn)
infrared observations from 2004
2005, and constructed the
light curve of the outburst at 10 different wavelengths as well
as pre-outburst, outburst, and post-outburst spectral energy
distributions. Our results show that by 2004 OO Ser had already
returned to the pre-outburst state at 12 um and below, suggesing
a significantly shorter outburst timescale than that of classical
FUors, but longer than that of the EXors. This may indicate, that
OO Ser - together with the recently erupted star V1647 Ori - is
the first example of an intermediate object between FUors and EXors.