  We have observed again two stars inside the ZZ Ceti instability strip that
were previously classified as not-observed-to-vary (NOV) by Mukadam et al.
(2004) and found them to be low-amplitude variables. Some evidence points to a
pure ZZ Ceti instability strip; other evidence contests it. The two stars
previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic
effective temperatures that place them inside the ZZ Ceti instability strip,
and they were "contaminating" the strip as constant stars, which could indicate
that the instability strip was no longer a simple evolutionary stage. A pure
instability strip indicates that pulsation is a normal phase which all DAs must
go through. We used effective temperatures derived from SDSS optical spectra by
comparing them with model atmospheres to look for pulsators through
time-resolved photometry and stars previously classified as NOV. Our new
results indicate, but do not prove, a pure instability strip, because there are
still other NOV stars that need to be observed again. Additionally, we have
discovered five other ZZ Ceti stars based on their effective temperatures.
