  FU Orionis-type objects (FUors) are low-mass young eruptive stars that
represent an evolutionary phase characterized by episodic periods of increased
accretion rate from the disk to the star. Theory predicts that a circumstellar
envelope, the source of continuous mass infall onto the disk, is necessary for
triggering such accretion bursts. We study the spatial and velocity structure
of envelopes around FUors by means of molecular line observations at mm
wavelengths. We target three prototypical FUors and an object possibly in a
pre-outburst state. We present archival PdBI interferometric observations of
the J=1-0 line of 13CO at 110.2 GHz. For three of our targets, these represent
the first mm interferometric observations. The data allow the study of the
molecular environment of the objects on a spatial resolution of a thousand AU
and a velocity resolution of 0.2 km/s. Strong, narrow 13CO(1-0) line emission
is detected from all sources. The emission is spatially resolved in all cases,
with deconvolved sizes of a few thousand AUs. For V1057 Cyg and V1331 Cyg, the
emitting area is rather compact, suggesting that the origin of emission is an
envelope surrounding the central star. For V1735 Cyg, the 13CO emission is
offset from the stellar position, indicating that the source of emission may be
a small foreground cloud, also responsible for the high reddening of the
central star. The 13CO emission towards V1515 Cyg is the most extended in the
sample, and coincides with the ring-like optical reflection nebula associated
with V1515 Cyg. We suggest that mm interferometric observations are
indispensable for a complete understanding of the circumstellar environment of
FUors. Any theory of the FUor phenomenon that interprets the geometry of the
circumstellar structure and its evolution using single beam measurements must
be checked and compared to interferometric observations in the future.
