  Drift pairs are an unusual type of fine structure sometimes observed in
dynamic spectra of solar radio emission. They appear as two identical short
narrowband drifting stripes separated in time; both positive and negative
frequency drifts are observed. Using the Low Frequency Array (LOFAR), we report
unique observations of a cluster of drift pair bursts in the frequency range of
30-70 MHz made on 12 July 2017. Spectral imaging capabilities of the instrument
have allowed us for the first time to resolve the temporal and frequency
evolution of the source locations and sizes at a fixed frequency and along the
drifting pair components. Sources of two components of a drift pair have been
imaged and found to propagate in the same direction along nearly the same
trajectories. Motion of the second component source is delayed in time with
respect to that of the first one. The source trajectories can be complicated
and non-radial; positive and negative frequency drifts correspond to opposite
propagation directions. The drift pair bursts with positive and negative
frequency drifts, as well as the associated broadband type-III-like bursts, are
produced in the same regions. The visible source velocities are variable from
zero to a few $10^4$ (up to ${\sim 10^5}$) km/s, which often exceeds the
velocities inferred from the drift rate ($\sim 10^4$ km/s). The visible source
sizes are of about $10'-18'$; they are more compact than typical type III
sources at the same frequencies. The existing models of drift pair bursts
cannot adequately explain the observed features. We discuss the key issues that
need to be addressed, and in particular the anisotropic scattering of the radio
waves. The broadband bursts observed simultaneously with the drift pairs differ
in some aspects from common type III bursts and may represent a separate type
of emission.
