On statistical arguments FU Ori eruptions must be very common among
 pre-main-sequence (PMS) stars, yet only a few cases are known.
 Consequently, this phase of PMS evolution is poorly understood. A little
 bit more is known about EX Lup outbursts because some cases are known
 where the same star has undergone more than one event. Nevertheless, the
 physical reasons for an outburst are not known. Two models have been
 presented: rotation induced mass outflow and enhanced disk accretion.
 The biggest predicted observable difference is in the infrared. The
 accretion model predicts infrared brightening prior to the maximum
 activity while outflow model is consistent with infrared brightening
 after the main activity phase. A simple infrared photometric monitoring
 will discriminate between the models and provide unique data necessary
 to understand the physics involved. The rarity of outbursts triggers
 most ground-based observatories to monitor the process. If an eruption
 occurs when ISO is operational, it has to be recorded also in the
 wavelengths where majority of the luminosity is emitted. This ToO
 programme proposes the most basic monitoring measurements for an
 erupting PMS star.