  We explain theoretically peculiarities of the smectic A -- hexatic B
equilibrium phase coexistence in a finite temperature range recently observed
experimentally in free standing smectic films [I.A.Zaluzhnyy et al., Physical
Review E, volume 98, 052703 (2018)]. We quantitatively describe this unexpected
phenomenon within Landau phase transitions theory assuming that the film state
is close to a tricritical point. We found that the surface hexatic order leads
to diminishing the phase coexistence range as the film thickness decreases
shrinking it at some minimal film thickness, of the order of the hexatic
correlation length. We established universal laws for the temperature width of
the phase coexistence in terms of the reduced variables. Our theory is in
agreement with the existing experimental data.
