Weighing a galaxy bar in the lens Q2237+0305

In the gravitational lens system Q2237+0305 the cruciform quasar image geometry is twisted by ten degrees by the lens effect of a bar in the lensing galaxy. This effect can be used to measure the mass of the bar. We construct a new lensing model for this system with a power-law elliptical bulge and a Ferrers bar. The observed ellipticity of the optical isophotes of the galaxy leads to a nearly isothermal elliptical profile for the bulge with a total quasar magnification of 16^{+5}{-4}. We measure a bar mass of 7.5\pm1.5 \times 10^8 h^{-1}{75} M_{\sun} in the region inside the quasar images.