Context :

Aims : In order to study the slope and strength of the non-stellar continuum , we analysed a sample from nearby Narrow Line Seyfert 1 ( NLS1 ) .
Also , we re-examined the location of NLS1 galaxies on the M _ { BH } - \sigma _ { \star } relation , using the stellar velocity dispersion and the [ OIII ] \lambda 5007 emission line as a surrogate of the former .

Methods : We studied spectra of a sample of 131 NLS1 galaxies taken from the Sloan Digital Sky Survey ( SDSS ) DR7 .
We approached determining the non-stellar continuum by employing the spectral synthesis technique , which uses the code starlight , and by adopting a power-law base to model the non-stellar continuum .
Composite spectra of NLS1 galaxies were also obtained based on the sample .
In addition , we obtained the stellar velocity dispersion from the code and by measuring Calcium II Triplet absorption lines and [ OIII ] emission lines .
From Gaussian decomposition of the H \beta profile we calculated the black hole mass .

Results : We obtained a median slope of \beta = - 1.6 with a median fraction of contribution of the non-stellar continuum to the total flux of 0.64 .
We determined black hole masses in the range of log ( M _ { BH } /M _ { \odot } ) = 5.6 - 7.5 , which is in agreement with previous works .
We found a correlation between the luminosity of the broad component of H \beta and black hole mass with the fraction of a power-law component .
Finally , according to our results , NLS1 galaxies in our sample are located mostly underneath the M _ { BH } - \sigma _ { \star } relation , both considering the stellar velocity dispersion ( \sigma _ { \star } ) and the core component of [ OIII ] \lambda 5007 .

Conclusions :