Context : FU Orionis-type objects are pre-main sequence , low-mass stars with large outbursts in visible light that last for several years or decades .
They are thought to represent an evolutionary phase during the life of every young star when accretion from the circumstellar disk is enhanced during recurring time periods .
These outbursts are able to rapidly build up the star while affecting the physical conditions inside the circumstellar disk and thus the ongoing or future planet formation .
In many models infall from a circumstellar envelope seems to be necessary to trigger the outbursts .

Aims : We characterize the morphology and the physical parameters of the circumstellar material around FU Orionis-type stars using the emission of millimeter wavelength molecular tracers .
The high spatial resolution study gives insight into the evolutionary state of the objects , the distribution of parameters in the envelopes and the physical processes forming the environment of these stars .

Methods : We observed the J=1 - 0 rotational transition of ^ { 13 } CO and C ^ { 18 } O towards eight northern FU Orionis-type stars ( V1057 Cyg , V1515 Cyg , V2492 Cyg , V2493 Cyg , V1735 Cyg , V733 Cep , RNO 1B and RNO 1C ) and determine the spatial and velocity structure of the circumstellar gas on the scale of a few thousands of AU .
We derive temperatures and envelope masses and discuss the kinematics of the circumstellar material .

Results : We detected extended CO emission associated with all our targets .
Smaller scale CO clumps were found to be associated with five objects with radii of 2000 - 5000 AU and masses of 0.02 - 0.5 M _ { \odot } ; these are clearly heated by the central stars .
Three of these envelopes are also strongly detected in the 2.7 mm continuum .
No central CO clumps were detected around V733 Cep and V710 Cas that can be interpreted as envelopes but there are many other clumps in their environments .
Traces of outflow activity were observed towards V1735 Cyg , V733 Cep and V710 Cas .

Conclusions : The diversity of the observed envelopes enables us to set up an evolutionary sequence between the objects .
We find their evolutionary state to range from early , embedded Class I stage to late , Class II-type objects with very low-mass circumstellar material .
We also find evidence of larger scale circumstellar material influencing the detected spectral features in the environment of our targets .
These results reinforce the idea of FU Orionis-type stars as representatives of a transitory stage between embedded Class I young stellar objects and classical T-Tauri stars .