  Scattering amplitudes in a range of quantum field theories can be computed
using the Cachazo-He-Yuan (CHY) formalism. In theories with colour ordering,
the key ingredient is the so-called Parke-Taylor factor. In this note we give a
fully $\text{SL}(2,\mathbb{C})$-covariant definition and study the properties
of a new integrand called the string Parke-Taylor factor. It has an $\alpha'$
expansion whose leading coefficient is the field-theoretic Parke-Taylor factor.
Its main application is that it leads to a CHY formulation of open string
tree-level amplitudes. In fact, the definition of the string Parke-Taylor
factor was motivated by trying to extend the compact formula for the first
$\alpha'$ correction found by He and Zhang, while the main ingredient in its
definition is a determinant of a matrix introduced in the context of string
theory by Stieberger and Taylor.
