Thanks for your response!
When I place an oscillator probe at pin 6 (threshold pin) i see something strange. The output at pin 3 is supposed to trigger from High to low when the Voltage at pin 6 is 2/3 Vcc (8 V). however the output actually triggers when the voltage across capacitor C2 reaches 10 V. As a result I get a very high on time (70s) compared to off time ( 30s). Is this also due to leakage of capacitor?
I am switching in extra capacitors to get different time constants.