  We study formation of 9:7 mean motion resonance (MMR) as a result of
convergent migration of two planets embedded in a disc. Depending on the
migration parameters, initial orbits and planets' masses the system may pass
through the resonance or enter it (permanently or temporarily). We illustrate
that a stable equilibrium of the averaged system (a periodic orbit of the
N-body model) is surrounded by the region of permanent resonance capture, whose
size depends on the migration parameters and the planets mass ratio. A system
located inside this region tends towards the equilibrium (the capture is
permanent), while a system located outside the region evolves away from the
equilibrium and leaves the resonance. We verify recent results of Delisle et
al. and Xu & Lai where they show that for $m_1 \lesssim m_2$ ($m_1, m_2$ are
the inner and outer planets' masses) the equilibrium is unstable when the
migration is added, so the capture cannot be permanent. We show that for
particular migration parameters the situation may be reversed (the equilibria
are unstable for $m_1 \gtrsim m_2$). We illustrate that 9:7 MMR consists of two
modes separated with a separatrix. The inner one is centred at the equilibrium
and the outer one has no equilibrium in its centre. A system located outside
the region of stable capture evolves from the inner into the outer mode. The
evolution occurs along families of periodic orbits of the averaged system, that
play a crucial role in the dynamics after the resonance capture.
