The extended HI rotation curve and mass distribution of M31

New H I observations of Messier 31 (M31) obtained with the Effelsberg and Green Bank 100 m telescopes make it possible to measure the rotation curve of that galaxy out to similar to 35 kpc. Between 20 and 35 kpc, the rotation curve is nearly flat at a velocity of similar to 226 km s(-1). A model of the mass distribution shows that at the last observed velocity point, the minimum dark-to-luminous mass ratio is similar to 0.5 for a total mass of 3.4 x 10(11) M-circle dot at R < 35 kpc. This can be compared to the estimated Milky Way mass of 4.9 x 10(11) M-circle dot for R < 50 kpc.