Question
I live in Central VA, my front lawn is fescue, south facing with a gentle slope, and full sun all day in the summer.  The soil is clay, pH is good, but I do not have an irrigation system.  The fescue pretty much goes dormant with the first heat wave, and by the end of the summer I have large 'dead' patches.  Some come back, but not fully.  Overseeding in the fall is problematic because we usually go on water restrictions after a hot dry summer, so we can't even water new seed in September, which is recommended for fescue.  I would like to sprig bermuda grass in the worst areas and let it fill in. Is this possible, or do I have to kill off the existing grass first?  Once the bermuda establishes itself, will it choke out the fescue or will the fescue return in the fall?  If the fescue gets choked out, I was thinking about overseeding in the late fall like a golf course.  In this case, is annual or perennial ryegrass a better choice?  

For my area, is there a preferred variety of bermuda?  I believe there are some that do not tolerate cold so well, and we can have winters where the lawn has snow/ice and temperatures in the single digits.  

Thank you for your help.

Answer
Please forgive my indirect responses because the project you wish to undertake is like a web of little components which must come together rather like an orchestra - players and instruments - which will produce a symphony of sound. And you get to be the Conductor!
You do not have an irrigation system . Sprigs can fail completely if they cannot be watered at the required frequency. Follow this link for more information.
You wish to establish Bermuda . Here again, I will refer you to this link for the relevant information you need to make a clear decision. My suggestion is to go for a "common" Bermuda seed. Improved varieties tend to be more fitted to 'golf-course' rather than 'homeowner' program of maintenance.
Choose carefully. It can challenge your time-management skills.