Looking to modify this circuit for 30Hz operation

I was given the parts to build this sine wave generator. As far as I
can tell it's a Wein Bridge with the two LEDs serving the function
originally reserved for the incandescent lamp.

http://www.stephenhobley.com/sinewave.gif

Unfortunately it's fixed frequency, the 5K variable only trims one
"side" of the wave, not selecting the fundamental as I'd hoped. It
looks liket the 68K resistors and 0.047uF caps determine the period of
oscillation (one pair for each half).

Does anyone know the mathmatical relationship between the values of R5/
R6 and C1/C2 and the frequency of oscillation? I'd like to get this
working in the 30Hz range.

Thanks,
Steve
.