  Phase locking experiments on vortex based spin transfer oscillators with an
external microwave current are performed. We present clear evidence of phase
locking, frequency pulling, as well as fractional synchronization in this
system, with a minimum peak linewidth of only 3 kHz in the locked state. We
find that locking ranges of the order of 1/3 of the oscillator frequency are
easily achievable because of the large tunability $\partial f/\partial I_{dc}$
observed in our vortex based systems. Such large locking ranges allow us to
demonstrate the simultaneous phase locking of two independent oscillators
connected in series with the external source.
