  We investigate the Mach number dependence of the radiative overstability of
shocks. We find that the stability of radiative shocks increases with
decreasing Mach number, with the result that M=2 shocks require cooling
exponents < -1.2 to be overstable. The frequency of oscillation of the
fundamental mode has a strong Mach number dependence. We find that feedback
between the cooling region and the cold dense layer (CDL) further downstream is
a function of Mach number, with stronger feedback and oscillation of the
boundary between the CDL and the cooling region occuring at lower Mach numbers.
This feedback can be quantified in terms of the reflection coefficient of sound
waves. An interesting finding is that the stability properties of low Mach
number shocks can be dramatically altered if the shocked gas is able to cool to
temperatures less than the pre-shock value, and it is probable that low Mach
number astrophysical shocks will be overstable in a variety of situations. The
results of this work are relevant to astrophysical shocks with low Mach
numbers, such as supernova remnants (SNRs) immersed in a hot interstellar
medium (e.g., within a starburst region), and shocks in molecular clouds, where
time-dependent chemistry can lead to overstability. (Abridged)
