  Gaia21bty, a pre-main sequence star that previously had shown aperiodic dips
in its light curve, underwent a considerable $\Delta G\approx2.9$ mag
brightening that occurred over a few months between 2020 October - 2021
February. The Gaia lightcurve shows that the star remained near maximum
brightness for about $4-6$ months, and then started slowly fading over the next
2 years, with at least three superimposed $\sim$1 mag sudden rebrightening
events. Whereas the amplitude and duration of the maximum is typical for EXors,
optical and near-infrared spectra obtained at the maximum are dominated by
features which are typical for FUors. Modelling of the accretion disc at the
maximum indicates that the disc bolometric luminosity is 43 L$_{\odot}$ and the
mass accretion rate is $2.5\times10^{-5}$ M$_{\odot}$ yr$^{-1}$, which are
typical values for FUors even considering the large uncertainty in the distance
($1.7_{-0.4}^{+0.8}$ kpc). Further monitoring is necessary to understand the
cause of the quick brightness decline, the rebrightening, and the other
post-outburst light changes, as our multi-colour photometric data suggest that
they could be caused by a long and discontinuous obscuration event. We
speculate that the outburst might have induced large-scale inhomogeneous dust
condensations in the line of sight leading to such phenomena, whilst the FUor
outburst continues behind the opaque screen.
