  We report some remarkable and unexpected results of optical/UV/X-ray
follow-up observations of bright soft X-ray selected AGN from the ROSAT
All-Sky-Survey. The majority of these AGN are rather anonymous Seyfert
galaxies, mostly of the narrow-line Seyfert 1 subtype (nlSy1). We confirm the
well-known X-ray variability by factors of a few. However, we also found
strikingly different variability patterns: (i) a drop in the PSPC count rate by
a factor of ~400 in WPVS007, (ii) a bolometrically dominant soft X-ray
component decreasing in flux by a factor ~100 in IC 3599 accompanied by a
decrease in optical emission line fluxes, and (iii) a drastic X-ray spectral
change in RX J0134-42 from ultrasoft to a typical hard Seyfert spectrum. These
dramatic variations occur within a (few) year(s), implying that the accretion
flow in the immediate vicinity of the central black hole must have undergone a
major change. We discuss possible physical explanations such as accretion disk
instabilities or the tidal disruption of stars.
