  We discuss the effects of energy injection into the intra-group medium of the
group of galaxies associated with NGC 741. The X-ray emission reveals a large
bubble, which in the absence of a currently bright central radio source, may
have been inflated by a previous cycle of nuclear activity . If the bubble is
filled with a light, relativistic fluid we calculate that in expanding, it has
done more than sufficient work to counteract the energy lost from the
intra-group medium via radiative cooling; the bubble can provide this energy as
it expands and rises. Using upper limits on the flux density of the plasma
filling the bubble at 330 MHz and 1.4 GHz, we derive constraints on its
electron energy distribution and magnetic field strength. We show that the data
require the high-energy cut-off of the electron spectrum to be very low
compared to the cut-offs seen in more typical radio sources if the fluid
filling the bubble is a conventional relativistic plasma. This suggests that
the fluid filling the bubble may not have evolved by expansion or synchrotron
losses consistent with a dead radio source, leaving a puzzle as to what the
origin of the bubble may be.
