SIMBAD references

We present the results of a monitoring program on a sample of `Narrow Line Seyfert 1 Galaxies': this subclass of Seyfert 1's is characterized by very low FWHMs of the permitted lines (≲1000km/s), and variability can help discriminate among the several intepretative models for this behaviour. We found that, out of 12 objects, 10 of them displayed significant variations in the flux of the optical permitted lines over a time interval of one year; the comparison with a similar data-set of `normal' Seyfert 1's does not show statistical differences between the variability properties of the two classes. Some of the possible models are discussed, also on the basis of the observational constraints. A shorter time-scale monitoring would be necessary to discriminate between the most promising models.