This expression will match Eq. 24 only if σ$_{z}$ = σ$_{R}$ , which is not generally the case ( § 7.2). Both expressions reduce the asymmetric drift relative to what Eq. 3 predicts; in particular, the addition of an inner truncation reduces the expected drift significantly in the inner parts. The size of the change is only a few km s − $^{1}$, but it is still a significant fraction of the predicted drift. This is seen in Fig. 22, in which the effects of adding an inner truncation and spherical alignment of the velocity ellipsoid are examined. It is noteworthy that the inner truncation can effectively wipe out the asymmetric drift over a large range in radius; a similar effect is seen in Fig. 5. For Fig. 22, we use the fiducial values for scenario K found in Table 2. Note that the increased asymmetric drifts at larger radii are due to the large R$_{σ}$ of this scenario, and the asymmetric drift expression breaks down inside ∼ 0 . 5 R$_{d}$ . We test cases in which R$_{σ}$ is reduced to 50% of the fiducial value and find that the asymmetric drift is reduced in the outer part. Thus, as is evident from Fig. 22, a combination of factors can serve to reduce the asymmetric drift nearly to the observed values in this galaxy. One should keep in mind that the sensitivity of the asymmetric drift expression to such changes means that the predicted drifts may fluctuate wildly depending on the assumptions. Because of these issues, the circular velocity derived in scenarios K, KL and E may be larger than the true circular velocity. These caveats further suggest that the asymmetric drift found in scenario KG may be smaller than seen in Fig. 5, and hence that scenario KG may be potentially viable. Without more information about the asymmetric drift in real galaxies, it is impossible to claim that scenario KG is definitely unrealistic; however, scenarios K and E remain our preferred models for this galaxy. Recent work by Freeland et al. (2010) suggests that NGC 6503 may possess a nuclear disc of radius ∼ 100 pc and an end-on bar; deprojection of their H − band image of NGC 6503 shows a bar like structure perpendicular to the major axis with clear spiral arms emanating from the ends. This scenario is consistent with our scenario E. It would moreover explain the σ − drop because of star formation at the centre caused by bar induced gas inflow and because diagnostics from Bureau & Athanassoula (2005) suggest that end-on bars may generate σ − drops. These findings are compelling evidence for a bar in NGC 6503. Notably, these authors also estimate the age of the star formation ring at ∼ 0 . 5 Gyr, which is well below the timescale for bar destruction determined by Bournaud et al. (2005) of ∼ 1 Gyr. The presence of a bar in NGC 6503 is unlikely to change our main conclusions because it is end on. There is no clear kinematic indicator of bar structure in the stellar or ionized gas rotation curve; we expect that the measured values are largely unaffected by the bar 28 7.4. Evidence for a bar