Monotonicity of some functions involving the gamma and psi functions

Let L(x) := x - Gamma(x+t)/Gamma(x+s)x(s-t+1), where Gamma(x) is Euler's gamma function. We determine conditions for the numbers s, t so that the function Phi(x) := -Gamma(x+s)/Gamma(x+t)x(t-s-1) L ''(x) is strongly completely monotonic on (0, infinity). Through this result we obtain some inequalities involving the ratio of gamma functions and provide some applications in the context of trigonometric sum estimation. We also give several other examples of strongly completely monotonic functions defined in terms of Gamma and psi:= Gamma'/Gamma functions. Some limiting and particular cases are also considered.