2005A&A...436...91D

We employ the CFHT Megacam camera to survey ∼one deg² of the southern outer disc of M31, a region which includes the area where Battinelli et al. (2003AJ....125.1298B) have identified nearly one thousand C stars. In the outer M31 region not previously surveyed, we identify 361 new C star candidates, having similar photometric properties to the known ones, and confirm the slight decrease in the luminosity of C stars with galactocentric distances. We show that the Sloan g', r', i' filters are a viable approach, comparable to (CN-TiO), to identify C stars. We find that the (g'-r') colours of cool C stars can be so red that prohibitively long g' exposures are needed to acquire faint extragalactic C stars. This makes the Sloan filters a less promising approach to extend a C star survey to several Mpc. Our uniform large field survey detects the edge of M31 disk at ∼35kpc. The intermediate-age population, represented by C stars, extends further to ∼40kpc.