few of the lowest-mass members of compact groups lie well “above” the relationship. If they once had the T-F parameters similar to galaxies in less dense environment, they could have either brightened by 1 to 2 magnitudes in the $B$ band, to get to their present position, they could have lost a substantial amount of mass, or they could have undergone a combination of these processes.

A natural way that the smaller members could have brightened is by forming stars, induced by the interactions the galaxy may have suffered within the group. For a given rate of star formation, the brightening of a galaxy will be much more noticeable for a low-mass than for a high-mass galaxy. As an example, for a large spiral galaxy with mass of 10$^{11}$ solar masses, an episode of star formation that generates, say, 10 solar masses per year, would increase the luminosity of the galaxy in the $B$ band only by about 20%, while for a galaxy with a mass ten times lower, the same episode of star formation would triple the luminosity of the galaxy, enhancing its magnitude by 1.2 mag. Moreover, the two galaxies in the lowest-mass end of the diagram (HCG 