  The effect of a time dependent cosmological constant is considered in a
family of scalar tensor theories. Friedmann-Robertson-Walker cosmological
models for vacumm and perfect fluid matter are found. They have a linear
expansion factor, the so called coasting cosmology, the gravitational
"constant" decreace inversely with time; this model satisfy the Dirac
hipotesis. The cosmological "constant" decreace inversely with the square of
time, therefore we can have a very small value for it at present time.
