variable 555 circuit with equal LOW&HIGH frequency
Dave Tweed has indicated a simple solution (+1 from me). The circuit for which is...

By charging and discharging the capacitor (C) through the resistance connected to the output (pin 3) the charge and discharge times are the same. By using a fixed resistor (R1) you can set the minimum time for the astable. By adding a variable resistor (VR1) you can easily alter the time without altering the mark/space ratio.  The voltage across the capacitor (C) charges and discharges between 1/3 and 2/3rds of the supply voltage set by the internal comparators of the 555. The ON time = OFF time = T = 0.7CR (where R = R1 + VR1) - C in Farads, R in Ohms, T in Seconds.