  We consider a molecular single electron transistor coupled to a vibrational
mode. For some values of the bias and gate voltage transport is possible only
by absorption of one ore more phonons. The system acts then as a cooler for the
mechanical mode at the condition that the electron temperature is lower than
the phonon temperature. The final effective temperature of the vibrational mode
depends strongly on the bias conditions and can be lower or higher of the
reservoir in contact with the oscillator. We discuss the efficiency of this
method, in particular we find that there is an optimal value for the
electron-phonon coupling that maximizes cooling.
