of the observations and data reduction can be found in Mendes de Oliveira et al. (1998), Plana et al. (2003) and Amram et al. (2003). In summary, our dataset consists of H$\alpha$ emission-line velocity maps obtained with a Fabry-Perot system, from which rotation curves were obtained. For the T-F study we excluded from the sample all the elliptical galaxies. We also excluded galaxies HCG 10a, 10c, 16d and 100a because their rotation curves have a very short extension, well short of R$_{25}$ and galaxies HCG 16b and HCG 100b due to their extremely peculiar rotation curves. We included HCG 19a in our sample, although Hickson (1993) classifies it as an E2 galaxy, because we have reclassified it as an S0 based on its kinematic properties. In addition, we included in our sample unpublished data for two other galaxies (HCG 07c and HCG 79d).

We show in Figs. 1a and 1b the rotation curves for the galaxies in the sample studied in this paper. The x-axis plots r/R$_{25}$, the galaxy radius along its major axis normalized by R$_{25}$, the length of the major axis at the 25 mag arcsec$^{-2}$ isophote (as given by Hickson 1993; note, however that in Plana et 