  An interacting pair of chemotactic (anti-chemotactic) active colloids, that
can rotate their axes of self-propulsion to align {parallel (anti-parallel)} to
a chemical gradient, shows dynamical behaviour that varies from bound states to
scattering. The underlying two-body interactions are purely dynamical,
non-central, non-reciprocal, and controlled by changing the catalytic activity
and phoretic mobility. Mutually chemotactic colloids trap each other in a final
state of fixed separation; the resulting `active dimer' translates. A second
type of bound state is observed where the polar axes undergo periodic cycles
leading to phase-synchronised circular motion around a common point. These
bound states are formed depending on initial conditions and can unbind on
increasing the speed of self propulsion. Mutually anti-chemotactic swimmers
always scatter apart. We also classify the fixed points underlying the bound
states, and the bifurcations leading to transitions from one type of bound
state to another, for the case of a single swimmer in the presence of a
localised source of solute.
