Eruptive young stars are believed to constitute a sub-class of Pre-Main Sequence stars, which due to enhanced accretion from their circumstellar disks, experience fast increases in brightness in the visual and near-IR. This sub-class is divided into two groups, the EX Lupi-type (EXors) and the FU Ori-type (FUors), of which the latter is named after the archetype FU Orionis. FUors experience a rapid brightening (as high as Δm_B~6 mag) that can last from a few months to years, caused sudden spikes in mass accretion rates as high as 1e-4 Msun/year, followed by a very slow dimming. There are about 20 FUors known thus far, while it is yet unclear whether they constitute a typical step in early stellar evolution. FUors are ideal laboratories in studying the evolution of disks within a few years after an eruptive event.
Due to their relative sizes (<=100 au) the inner portions of these disks are best observed with interferometric imaging instruments. The Very Large Telescope Interferometer (VLTI) now offers a new such instrument, MATISSE, that operates in the mid-infrared covering L, M, and N bands. Here, we present our recent work on observations of three FUors (Z CMa, V900 Mon, and FU Ori) with MATISSE/VLTI. We found that the regions of the protostellar disks emitting in the thermal infrared were much smaller compared to previous studies, which enforces stringent constraints on the size of the hot, inner region of the disks. For the particular case of FU Ori, our radiative transfer simulations suggest a flared disk with an inner radius r_in<1 au for its outer, silicate-rich region, while initial interferometric results suggest r_in<2 au and <7 au for V900 Mon and Z CMa SE, respectively. An analysis of new VLTI observations for at least two more eruptive stars in expected in the near future.