  Recently, an increasing number of wide binaries has been discovered. Their
chemical and dynamical properties are studied through extensive surveys and
pointed observations. However, the formation of these wide binaries is far from
clear, although several scenarios have been suggested. In order to investigate
the chemical compositions of these systems, we analysed high-resolution
spectroscopy of three wide binary pairs belonging to the Galactic halo. In
total, another three candidates from our original sample of 11 candidates
observed at various resolutions with various instruments were refuted as
co-moving pairs because their radial velocities are significantly different.
Within our sample of wide binaries, we found homogeneity amongst the pair
components in dynamical properties (proper motion and line-of-sight velocities)
and also in chemical composition. Their metallicities are -1.16, -1.42, and
-0.79 dex in [Fe/H] for each wide binary pair, which places these stars on the
metal-poor side of wide binaries reported in the literature. In particular, the
most metal-poor pair in our sample (WB2 = HD134439/HD134440) shows a lower
[$\alpha$/Fe] abundance ratio than Milky Way field stars, which is a clear
signature of an accreted object. We also confirmed that this wide binary shares
remarkably similar orbital properties with stars and globular clusters
associated with the Sequoia event. Thus, it appears that the WB2 pair was
formed in a dwarf galaxy environment and subsequently dissolved into the Milky
Way halo. Although the other two wide binaries appear to arise from a different
formation mechanism, our results provide a novel opportunity for understanding
the formation of wide binaries and the assembly process of the Milky Way.
