  The convergent migration of two planets in a gaseous disc can lead to capture
in mean motion resonance (MMR). In addition, pairs of planets in or near MMRs
are known to produce strong transit timing variations (TTVs). In this paper we
study the impact of disc-induced migrations on the TTV signal of pairs of
planets that enter a resonant configuration. We show that disc-induced
migration creates a correlation between the amplitude and the period of the
TTVs. We study the case of K2-24, a system of two planets whose period ratio
indicates that they are in or near the 2:1 MMR, with non-zero eccentricities
and large-amplitude TTVs. We show that a simple disc-induced migration cannot
reproduce the observed TTVs, and we propose a formation scenario in which the
capture in resonance occurring during migration in a disc with strong
eccentricity damping is followed by eccentricity excitation during the
dispersal of the disc, assisted by a third planet whose presence has been
suggested by radial velocity observations. This scenario accounts for the
eccentricities of the two planets and their period ratio, and accurately
reproduces the amplitude and period of the TTVs. It allows for a unified view
of the formation and evolution history of K2-24, from disc-induced migration to
its currently observed properties.
