  The earliest phases of star formation are characterised by intense mass
accretion from the circumstellar disk to the central star. One group of young
stellar objects, the FU Orionis-type stars exhibit accretion rate peaks
accompanied by bright eruptions. The occurrence of these outbursts might solve
the luminosity problem of protostars, play a key role in accumulating the final
star mass, and have a significant effect on the parameters of the envelope and
the disk. In the framework of the Structured Accretion Disks ERC project, we
are conducting a systematic investigation of these sources with millimeter
interferometry to examine whether they represent normal young stars in
exceptional times or they are unusual objects. Our results show that FU
Orionis-type stars can be similar to both Class I and Class II systems and may
be in a special evolutionary phase between the two classes with their
infall-driven episodic eruptions being the main driving force of the
transition.
