Physical parameters of three field RR Lyrae stars
  Str\"omgren $uvby-\beta$ photometry of the stars classified as RR Lyrae stars
RU Piscium, SS Piscium and TU Ursae Majoris has been used to estimate their
iron abundance, temperature, gravity and absolute magnitude. The stability of
the pulsating period is discussed. The nature of SS Psc as a RRc or a HADS is
addressed. The reddening of each star is estimated from the Str\"omgren colour
indices and reddening sky maps. The results of three approaches to the
determination of [Fe/H], $T_{\rm eff}$ and $\log(g)$ are discussed: Fourier
light curve decomposition, the Preston $\Delta S$ index and the theoretical
grids on the $(b-y)_o - c_1{_o}$ plane.
