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 \sim 35 kpc .
Between 20 and 35 kpc , the rotation curve is nearly flat at a velocity of \sim 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 \sim 0.5 for a total mass of 3.4 \times 10 ^ { 11 } { \cal M } _ { \odot } at R < 35 kpc .
This can be compared to the estimated MW mass of 4.9 \times 10 ^ { 11 } { \cal M } _ { \odot } for R < 50 kpc .