  In this paper we study the phonon's effect on the position of the 1s
excitonic resonance of the fundamental absorption transition line in
two-dimensional transition metal dichalcogenides. We apply our theory to
WS$_{2}$a two-dimensional material where the shift in absorption peak position
has been measured as a function of temperature. The theory is composed of two
ingredients only: i) the effect of longitudinal optical phonons on the
absorption peak position, which we describe with second order perturbation
theory; ii) the effect of phonons on the value of the single particle energy
gap, which we describe with the Huang Rhys model. Our results show an excellent
agreement with the experimentally measured shift of the absorption peak with
the temperature.
