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Published online by Cambridge University Press: 25 May 2016
We have obtained counts of stars near the tip of the red giant branch of M31, and have used these counts to estimate the surface brightness of the halo of M31 down to a level of μV ∼ 30 mag arcsec–2 (R ∼ 20 kpc). The surface brightness along the minor axis of the M31 halo is well-represented by a single de Vaucouleurs law (0.2 ≃ R[kpc] ≃ 20). Alternatively, the outer halo of M31 can also be modelled by a power-law density distribution of the form ρ(R) ∞ R –5. This result suggests that the globular cluster component of the halo of M31 (for which ρ ∞ R –3) is more extended than the stellar halo of this galaxy. At μv à 28 mag arcsec–2 (R à 10 kpc), the axial ratio of the halo of M31 is found to be c/a = 0.55 ± 0.05.