R4 and R5 provide separate charge and discharge paths for the capacitor C depending on the output state.
The charge path is set through R5 and D2 when the output is high. Similarly, the discharge path for the capacitor is set by R4 and D1 when the output is low.
The pulse width t1 is determined by the RC time constant of R5 and C. Thus, the time t2 between the pulses can be changed by varying R4, and the pulse width can be altered by R5. The frequency of the output can be changed by varying both R4 and R5. At low voltages, the effects of the diode forward drop (0.8 V, or 0.15 V for Shottky) must be taken into account by altering output high and low voltages in the calculations. RPU should be at least 10x less than the smallest value of R4 or R5.