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Context. Scalar-tensor gravity (STG) theories are well-motivated alternativesto general relativity (GR). One class of STG theories, theDamour-Esposito-Farese (DEF) gravity, has a massless scalar field with twoarbitrary coupling parameters. We are interested in this theory because,despite its simplicity, it predicts a wealth of different phenomena, such asdipolar gravitational wave emission and spontaneous scalarization of neutronstars (NSs). These phenomena of DEF gravity can be tested by timing binaryradio pulsars. Aims. We aim to develop a new binary pulsar timing model DDSTGto enable more precise tests of STG theories based on a minimal set of binaryparameters. The expressions for post-Keplerian (PK) parameters in DEF gravityare self-consistently incorporated into the model. The new technique takes intoaccount all possible correlations between PK parameters naturally. Methods.Grids of physical parameters of NSs are calculated in the framework of DEFgravity for a set of 11 equations of state. The automatic Differentiation(AutoDiff) technique is employed, which aids in the calculation ofgravitational form factors of NSs with higher precision than in previous works.The pulsar timing program TEMPO is selected as a framework for the realizationof the DDSTG model. The implemented model is applicable to any type of pulsarcompanions. Results. We apply the DDSTG model to the most recently publishedobservational data for PSR J2222-0137. The obtained limits on DEF gravityparameters for this system confirm and improve previous results. New limits arealso the most reliable because DEF gravity is directly fitted to the data. Weargue that future observations of PSR J2222-0137 can significantly improve thelimits and that PSR-BH systems have the potential to place the tightest limitsin certain areas of the DEF gravity parameter space.

A new pulsar timing model for scalar-tensor gravity with applications to PSR J2222-0137 and pulsar-black hole binaries