  Phase coupling between auto-oscillators is central for achieving coherent
responses such as synchronization. Here we present an experimental approach to
probe it in the case of two dipolarly coupled spin-torque vortex
nano-oscillators using an external microwave field. By phase-locking one
oscillator to the microwave field, we observe frequency pulling on the second
oscillator. From coupled Thiele equations we show analytically that this
frequency pulling results from concerted actions of dipolar and microwave
couplings. The analysis allows us to determine the amplitude of dipolar
coupling and the phase lag of each oscillator, yielding important information
for the implementation of large oscillator networks.
