We have measured the proper motion of the Local Group galaxy IC 10 with the Very Long Baseline Array by measuring the position of an H _ { 2 } O maser relative to two background quasars over 4.3 years .
After correction for the rotation of the Milky Way and IC 10 , we obtain a motion of –39 \pm 9 \mu as yr ^ { -1 } toward the East and 31 \pm 8 \mu as yr ^ { -1 } toward the North .
This corresponds to a total space velocity of 215 \pm 42 km s ^ { -1 } relative to the Milky Way for an assumed distance of 660 \pm 66 kpc .
Assuming that IC 10 and M33 , for which also a proper motion measurement exists , are bound to M31 , we calculate a lower limit for the mass of M31 of 7.5Â \times 10 ^ { 11 } M _ { \odot } .