Relaxation oscillator questions

[bograt]

hello,
i have recently been set a project to build a relaxation oscillator,
the build has been successful but i have now been set a new set of questions.

a: when the voltage is changed, which changes; the LED ON period, or the LED OFF period, or both? explain why and how could you modify the circuit to change this.

b: what happens to the frequency as the resistor value R is changed.

c: what happens when the capacitor value C is changed,

d: the relay has two sets of contacts but only one of the sets are used.
alter the circuit diagram to cause a second led to flash

any help with any of these questions would be greatly appreciated, i will try and include as much information as a can (below)

supply power:10v-15v
1x 12v relay with DPDT contacts
1x 0.5w resistor 100Ω (R in the diagram)
1x 0.5w resistor 220Ω
1x electrolytic capacitor 220µf (c in the circuit) minimum voltage handling 35v
1x LED
(build on veroboard)

hopefully the circuit diagram will work

thankyou all in advance, i really need help understanding this circuit.. simple as it may be.

regards