By varying the capacitor or resistor values in the positive feedback network of a Wien-Bridge Oscillator, the output could be Frequency Modulated.
Wien-Bridge oscillators in general are for low frequencies, up to 150kHz, and replacing the capacitors with varicaps is pretty hard, but resistors could be changed with FETs that can get at the gate input the modulation signal.
As a disadvantage of the WIEN-bridge oscillator a variation of one of the frequency determining parts within the positive feedback path does, at the same time, influence the oscillation condition. Thus, you have to vary two parts in parallel.
However, there are other alternatives for RC oscillators - in some cases with one-part tuning capabilities.