termines stability locally (Toomre 1964). As a purely phenomenological matter, Q can also determine global stability to bars (Athanassoula & Sellwood 1986). We test the stability of our models using N − body simulations. Our approach allows us to investigate numerous other properties of the galaxy. The S´ ersic index is indicative of bulge formation history (Courteau et al. 1996; Kormendy & Kennicutt 2004). We also explore the possibility that the bulge is related to luminous nuclear clusters found near the centres of many disc-dominated late-type galaxies, and derive M/L ratios for the disc and bulge. Moreover, we can constrain the cuspiness of the halo. Cosmological sim- ulations consistently demonstrate that halos are expected to have cuspy halos (e.g., Navarro et al. 1997; Moore et al. 1999), but there are considerable difficulties involved in inferring central halo densities from rotation curve measurements (Hayashi et al. 2004; Rhee et al. 2004; Dutton et al. 2005; Valenzuela et al. 2007). For example, gas rotation curve data near the centre of the galaxy may not trace the gravitational potential; using the stellar rota- tional velocity, where available, may be a better way to infer the cusp value (Pizzella et al. 2008). Further, noncircular gas motions at the centre (as may be caused by a triaxial halo) may skew the interpretation of the rotation curve, depending on the orientation of the disc (Simon et al. 2005). With both gas and stellar rotation curves, NGC 6503 provides an excellent opportunity to investigate these issues. This paper is organized as follows. In § 2 we detail the observational properties of NGC 6503 and elaborate on the peculiarities outlined above. In § 3 we discuss our dynamical models, and in § 4 we describe our application of the Bayes/MCMC method. We discuss our primary stability results in § 5, in the process constraining the available physics, and we present additional results in § 6. Section 7 is the discussion, which includes an exposition of prior research on NGC 6503. NGC 6503 is an isolated dwarf Sc galaxy at a distance of 5.2 Mpc (Karanchentsev & Sharina 1997) and is inclined at 74 ± 1 ◦ (Begeman 1987). It displays a small, bright central bulge, no signs of strong asymmetric central structure (e.g., no obvious bar) apart from some spirality, and it is mostly free of gas. In this section we review existing photometric and kinematic ob- servations of the galaxy. Throughout this paper, we use a distance at 5.2 Mpc and therefore 1 kpc = 40 ′′ at this distance. 4 2. NGC 6503: Observations and Issues