Astrophysics Problem (gravitational lensing)

[roam]

Homework Statement

I need some help with the following problem:

http://img543.imageshack.us/img543/9630/lensq.jpg

The Attempt at a Solution

First I have tried to work out the radius of the Einstein ring:

$r_E = \sqrt{\frac{4GM}{c^2}}.\sqrt{\frac{D_1D_{1s}}{D_s}} = \sqrt{\frac{4(6.67×10^{11})M)}{(3 \times 10^8)^2}}$

And here is a diagram of the situation under consideration:

http://img841.imageshack.us/img841/2751/ering.jpg

However I can't solve for r_E because I don't know M. But if I knew e_E I could find θ (the angle of deflection of light by a point mass) as follows:

$\theta = \frac{4GM}{c^2(r_E+r_s)}$

Where r_s is the stellar radius, and here is a diagram of the situation:

http://img542.imageshack.us/img542/8624/deflection.jpg

Where in this case I believe b = r_E + r_s.

So how can I solve for r_E and θ_E when I don't know what M is? Furthermore, how can I estimate the effective mass from the figure given in the question? Any help is greatly appreciated.

 

[roam]

My main question is, how can I find the mass of the lensing object? Without this I can't calculate anything else.