Lowes offers First Alert Photoelectric Combination Smoke and Carbon Monoxide Detector for $39.98 (reg $55) with free shipping.

Product Description:
The First Alert SMCO210 10-Year Sealed Battery Combination Smoke & Carbon Monoxide Alarm with Slim Profile Design uses First Alert's Precision Detection advanced sensing technology and meets new industry standards. This 2-in-1 alarm provides convenient protection against both fire and carbon monoxide (CO). Also equipped with a 10-year sealed battery to eliminate the need for battery replacements and low battery chirps.

• Precision Detection technology helps reduce cooking nuisance alarms and provides early warning.
• Provides reliable 2-in-1 protection against both smoke and carbon monoxide in an easy to install alarm
• 10-Year sealed battery eliminates battery replacements and low battery chirps for the life of the alarm
• End-of-life warning lets you know when it’s time to replace the alarm
• Slim profile design is half the depth of a standard alarm
• Test/silence button for efficient testing to ensure alarm is working properly