in the T-F diagram. We note that our values of V$_{max}$ are adjusted by the cosmological correction (1+z) as are also the values given for the galaxies in the control samples. All velocities were corrected for the same Virgocentric infall (Paturel et al. 1997) when distances were obtained using the Hubble law (for our sample and Courteau’s sample).

We note that in Fig. 1 the rotation of HCG 87a is one-sided because half of this almost edge-on galaxy is not observed due to a strong dust lane.

Comparisons with previous long-slit spectroscopy
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The 2-D velocity maps obtained with a Fabry-Perot instrument allowed us to mimic a slit through our data in order to recover the rotation curves obtained from long-slit spectroscopy. In this way, we could make a direct comparison with previous rotation curves obtained by other authors in order to investigate the reason for existing differences in the shapes and amplitudes of the rotation curves between different studies, for a given galaxy.

We give an example in Fig. 2, where it is clear why with long-slit spectroscopy the derived rotation curves do not always correspond to the overall kinematics of the galaxies. The rotation curve derived by R1991 