The ROSAT Ultradeep HRI survey in the Lockman Hole contains a complete sample of 91 X-ray sources with fluxes in the 0.5-2 keV band larger than 1.2 \cdot 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } , where over 75 % of the sources are quasars or Seyfert galaxies .
During the course of our optical identification work , we have obtained optical spectra of 67 narrow emission line galaxies ( NELG ) , which are physically not associated with the X–ray sources .
We have derived the equivalent width ( EW ) and the full width at half maximum ( FWHM ) for the most prominent emission lines of 41 quasars and Seyfert galaxies taken from the ROSAT Deep Survey ( RDS ) , which has a flux limit of 5.5 \cdot 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } in the 0.5-2.0 keV band .
Furthermore we have obtained the EW and FWHM values of the field NELGs .
Here we present the spectroscopic discrimination between RDS Seyfert galaxies and field galaxies ( NELG ) .
The analysis of the emission lines has revealed that a single object out of 69 spectroscopically identified AGN fits the optical criteria of Narrow-Line Seyfert 1 galaxies ( NLS1 ) .
This may indicate that NLS1 contribute only marginally to the soft X-ray background , but we can not exclude a possible larger contribution .