  We study the binding energies and optical properties of direct and indirect
excitons in monolayers and double layer heterostructures of Xenes: silicene,
germanene, and stanene. It is demonstrated that an external electric field can
be used to tune the eigenenergies and optical properties of excitons by
changing the effective mass of charge carriers. The Schr\"{o}dinger equation
with field-dependent exciton reduced mass is solved by using the Rytova-Keldysh
(RK) potential for direct excitons, while both the RK and Coulomb potentials
are used for indirect excitons. It is shown that for indirect excitons, the
choice of interaction potential can cause huge differences in the eigenenergies
at large electric fields and significant differences even at small electric
fields. Furthermore, our calculations show that the choice of material
parameters has a significant effect on the binding energies and optical
properties of direct and indirect excitons. These calculations contribute to
the rapidly growing body of research regarding the excitonic and optical
properties of this new class of two dimensional semiconductors.
