A Sino-German $\lambda$6\ cm polarization survey of the Galactic plane
  VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3
  Supernova remnants (SNRs) were often discovered in radio surveys of the
Galactic plane. Because of the surface-brightness limit of previous surveys,
more faint or confused SNRs await discovery. The Sino-German $\lambda$6\ cm
Galactic plane survey is a sensitive survey with the potential to detect new
low surface-brightness SNRs. We want to identify new SNRs from the $\lambda$6\
cm survey map of the Galactic plane. We searched for new shell-like objects in
the $\lambda$6\ cm survey maps, and studied their radio emission, polarization,
and spectra using the $\lambda$6\ cm maps together with the $\lambda$11\ cm and
$\lambda$21\ cm Effelsberg observations. Extended polarized objects with
non-thermal spectra were identified as SNRs. We have discovered two new, large,
faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure.
G178.2-4.2 has a size of 72 arcmin x 62 arcmin with strongly polarized emission
being detected along its northern shell. The spectrum of G178.2-4.2 is
non-thermal, with an integrated spectral index of $\alpha = -0.48\pm0.13$. Its
surface brightness is $\Sigma_{1 GHz} = 7.2 x 10^{-23}{Wm^{-2} Hz^{-1}
sr^{-1}}$, which makes G178.2-4.2 the second faintest known Galactic SNR.
G25.1-2.3 is revealed by its strong southern shell which has a size of 80
arcmin x 30\arcmin. It has a non-thermal radio spectrum with a spectral index
of $\alpha = -0.49\pm0.13$. Two new large shell-type SNRs have been detected at
$\lambda$6\ cm in an area of 2200 deg^2 along the the Galactic plane. This
demonstrates that more large and faint SNRs exist, but are very difficult to
detect.
