Another app note from ON Semiconductors on using PWM technique to reduce power consumed when latching mechanical relays. Link here (PDF)
Integrated circuit driver circuits often use relay loads in their application. Output drivers are a source of power dissipation on the IC. Latching relays can be used to keep sustaining load current at a minimum by engaging and removing drive current, but a PWM system can also preserve reduced power conditions by engaging and reducing duty cycle using standard type relays.
By considering the Maximum Turn−On Voltage and Minimum Turn−Off Voltage specifications typically quoted in the relay electrical specification, your system design can utilize a signal to pull−in and activate the relay followed by a reduced power PWM sustaining signal.