139 high-amplitude variable stars

The long mean baseline of 55yr along with a large sample of Class II YSOs (~15000) allows us to study approximately 1 million YSO-years. We find 139 objects with {Delta}R>=1mag, most of which are found at amplitudes between 1 and 3mag.

The majority of YSOs in this group show irregular variability or long-lasting fading events, which is best explained as hotspots due to accretion or by variable extinction. There is a tail of YSOs at {Delta}R>=3mag and they seem to represent a different population. Surprisingly many objects in this group show high-amplitude irregular variability over time-scales shorter than 10yr, in contrast with the view that high-amplitude objects always have long outbursts.