###### Abstract  
The most commonly used refrigerants are potent greenhouse gasses that can contribute to climate change. Hydro-Fluoro-Olefins are low Global Warming Potential fluids. A summary of our experimental research activity on the thermodynamic properties of two environmentally friendly Hydro-Fluoro-Olefins, namely R1234 yf and R1234 ze(E), is reported. In particular, the measurements were performed with an isochoric apparatus and the apparatus specifically built to reach temperatures down to about 100 K. The data elaboration confirms the validity of the choice and that R1234 yf and R1234 ze(E) can be adopted in many domestic applications. Moreover, considering the reduction of the flammability issues of R1234 yf and R1234 ze(E), the properties of binary systems containing these fluids and carbon dioxide were analyzed. The presented mixtures could be very interesting for low-temperature applications such as cascade cycles. G 20202010 March 202020020  
The challenge to find refrigerants that are at the same time low GWP, secure, and have a high performance is ongoing. [PERSON] et al. [6; 7] presented a well organised and comprehensive review of new environmentally friendly refrigerants, implementing a selection based on safety, environmental, and thermodynamic properties' criteria. They took into account several and important factors for the selection, such as low flammability, low GWP, efficiency, and proper capacity. The authors found a very limited group of compounds, most of them HydroFluoroOlefins (HFOs), that could be possible substitutes for old refrigerants. The HFOs have a carbon-carbon double-bound that causes them to have a very short lifetime in the atmosphere. Nevertheless, their disadvantages are that often, they are flammable or even toxic. Moreover, frequently, the thermophysical properties of these alternative refrigerants are not suitable for engineering applications. To fix these shortcomings, researchers are studying many blends of the former and the latter refrigerants in order to take into account the benefits of many compounds and balance their disadvantages [7]. Thanks to its thermodynamic properties, one of the most used refrigerants for domestic supplies is R134a [2]. As stated before, R134a has a very high level of GWP; therefore, the restriction of GWP 150 obliges the replacement in domestic refrigerator and freezers. Despite many of the pure HFOs and hydrocarbons being flammable, these alternatives to HFCs are being used in 90% of domestic use [8]. Hydrocarbons could be considered a very good alternative to R134a because of their high energy performance and could have several environmental advantages. However, in many countries, the safety regulations forbid the use of such compounds. For particular usages, such as domestic refrigeration, isobutane (R600a in the ASHRAE designation) could be considered as a good replacement for R134a [9]. For example, in Europe and Japan, the domestic regulations allow hydrocarbons such as R600a, despite many technical issues. But in many countries with more restrict safety regulations, this fluid is considered dangerous due to its flammability. For instance, in the United States, the amount of hydrocarbons for domestic use is restricted to 57 g for each charge, while in Europe, it is limited to 150 g [5]. For all these reasons, the fourth generation of synthetic refrigerants with a low GWP and low level of flammability has been arising a lot of interest by researchers and markets. In this study, a review of the thermodynamic properties of two of the most important HFOs, namely 2,3,3,3-tetrafluoroprop-1-ene (R1234 yf in the ASHRAE designation) and _trans_-1,3,3,3-tetrafluoropropene (R1234 ze(E) in the ASHRAE designation) is presented. The interest in these fluids characterized by GWP values near to 1 was confirmed by the [PERSON] et al. [10] review in which it was shown that the thermophysical properties of R1234 yf and R1234 ze(E) are the most investigated, also in terms of thermal conductivity, viscosity and surface tension.