If (&)references are alias of lvalue objects-variables then (&&)references are alias of what?

Question

References have no space in memory but they 'share' the same as their object. So If I did:

MyClass* obj=new MyClass();
MyClass& ref = *obj;

The ref would have the same adress as obj. However in this example:

std::string first= "Hello ";
std::string second="World!";
std::string&& ref= first+second;

What is the memory of ref?