Asymmetric rotation curves
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We can see in Fig. 1 that the rotation curves of HCG 91a (NGC 7214) and HCG96a (NGC 7674) present strong asymmetries, with one arm rising and the other one falling. Both HCG 91a and HCG 96a are the most massive members in M51-like pairs and they are also known to be Seyfert galaxies. We have checked the rotation curves of other most-massive galaxies in M51-like pairs in the sample of Márquez et al. (2002) and we have found several examples of similar rotation curves: NGC 3395/3396, NGC 5395/5394 and NGC 5774/5775. In fact, most galaxies in pairs of galaxies of unequal mass seem to have rotation curves with either one side rising and the other one falling or both falling. Examples of the latter class are NGC 4496a/4496b and NGC 2535/2536 (Amram et al. 1989). Other binary galaxies from Márquez’ sample show a rotation curve with high scatter and a trend for asymmetry (e.g. NGC 3769/3769A). Such rotation curves are rare among isolated galaxies. On the other hand, some rotation curves of galaxies of nearly-equal mass (e.g. NGC 5257/5258, Fuentes-Carrera 2003) curiously do not seem to show such peculiarities, presenting more symmetric rotation curves.

If 