  Hitherto, six P-type planets are found around five binary systems, i.e.
Kepler-16 b, 34 b, 35 b, 38 b, 47 b, c, which are all Neptune or Jupiter-like
planets. The stability of planets and the habitable zones are influenced by the
gravitational and radiative perturbations of binary companions. In this Letter,
we check the stability of an additional habitable Earth-mass planet in each
system. Based on our simulations in 10 Myr, a habitable "Earth" is hardly
stable in Kepler-16 while a stable "Earth" in Kepler-47 close to the boundaries
of the habitable zone is possible. In contrast, Kepler-34, 35 and 38 seem to
have high probabilities of being able to tolerate a stable "Earth" in their
habitable zones. The affects of transit time variations are quite small due to
the small mass of an undetected "Earth", except that of Kepler-16 b. With a
time precision of 0.001 day(~ 88 s), an "Earth" in the corotational resonance
with Kepler-16 b can be detected in 3 years, while habitable "Earths" in
Kepler-34 and 38 systems can be detected in 10 years. Habitable "Earths" in
Kepler-35 and 47 are not likely to be detected in 10 years under this
precision.
