  Lee-Yang and Fisher zeros are crucial for the study of phase transitions in
the grand canonical and the canonical ensembles, respectively. However, these
powerful methods do not cover the isothermal-isobaric ensemble (NPT ensemble),
which reflects the conditions of many experiments. In this work we present a
theory of the phase transitions in terms of the zeros of the NPT-ensemble
partition functions in the complex plane. The proposed theory provides an
approach to calculate all the partition function zeros in the NPT ensemble,
which form certain curves in the thermodynamic limit. To verify the theory we
consider Tonks gas and van der Waals fluid in the NPT ensemble. In the case of
Tonks gas, similarly to the Lee-Yang circle theorem, we obtain an exact
equation for the zero limit curve. We also derive an approximated limit curve
equation for van der Waals fluid in terms of the Szeg\"o curve. This curve fits
numerically calculated zeros and correctly describes how the phenomenon of
phase transition depends on the temperature.
