Purchasing duplex w/long term tenants who pay $1K under market

[Joan Smith]

Hi, 

We are purchasing a duplex. The current tenants have been there ten plus years are paying at least $1K under market and want to stay. They are month to month. They can not afford a $1K increase and I need to raise the rent at least $800 to cover expenses. I know the market as I have two other rentals in the town and in this market I could easily get $1,200- more than what they are paying. How can I increase the rent fairly? Have not seen the lease I am told they are month to month