The hidden nature of narrow line Seyfert 1 galaxies

Narrow-Line Seyfert 1 (NLS1) galaxies are relatively rare objects among type 1 nearby Active Galactic Nuclei (AGNs), and after almost two decades since their first classification, they are still a matter of debate. Their peculiar properties, like narrow permitted emission lines (FWHM < 2000 km/s), steep slopes and rapid variability in the soft X-ray domain, and the optical / UV Big Blue Bump shifted towards higher energy, are currently iinterpreted as indications of active nuclei younger than "classical" Seyfert 1 (S1) galaxies, powered by smaller supermassive black holes (M < 1E6 - 1E7 solar masses), and accreting at higher rates. To date this paradigm is not yet proved. Other possible scenarios were proposed and several authors challenged this topic from different points of view. Here we present a short review about the spectroscopic properties of NLS1s and recent results obtained within our group in investigating the BH-bulge relation of AGNs as a test for the NLS1-paradigm