For the past few months I have been studying $f(R)$ models of gravity. Recently I came cross $f(R, T)$ modified models of gravity. For the past few days I have tried to work out the difference between the two models. The problem is that while there are several models of $f(R)$ gravity, I couldn't find any models of $f(R,T)$ gravity. My question Are there any $f(R, T)$ models of gravity?