  We present new XMM-Newton observations of the hot-gas environments of two
low-power twin-jet radio galaxies, 3C 66B and 3C 449, showing direct evidence
for the interactions between X-ray-emitting gas and radio plasma that are
thought to determine the large-scale radio structure of these sources. The
temperatures that we measure for the two environments are significantly higher
than those predicted by standard luminosity-temperature relations for clusters
and groups. We show that luminosity-temperature relations for radio-quiet and
radio-loud X-ray groups differ, in the sense that radio-source heating may
operate in most groups containing radio galaxies. If the radio lobes are
expanding subsonically, we find minimum ages of 3e8 years for 3C 66B, and 5e8
years for 3C 449, older than the values obtained from spectral ageing, which
would give the radio source sufficient time to heat the groups to the observed
temperatures for plausible values of the jet power. The external pressures in
the atmospheres of both radio galaxies are an order of magnitude higher than
equipartition estimates of their radio-lobe pressures, confirming that the
radio lobes are either out of equipartition or require a pressure contribution
from non-radiating particles. Constraints from the level of X-ray emission we
measure from the radio lobes allow us to conclude that a departure from
equipartition must be in the direction of magnetic domination, and that the
most plausible candidates for a particle contribution to lobe pressure are
relativistic protons, an additional population of low-energy electrons, or
entrained and heated thermal material.
