Impossible to save

[Emmanuel Companys]

I my post I forgot to say that after I close the alert "can't save 
...", a file RR icon shows up with the title of the stack followed by 
the characters
.rev~
Then if I click on this icon I get
a) if  the RR program is not open :   "L'application Revolution a 
quitt inopinment" ("The application Revolution has unexpectedly 
quitted")
b) if Revolution is open: "Unable to open stack: stack is corrupted, 
check for ~ backup file."

What is this ' ~ backup file', and where is it?

Manuel