Are two calls to cout less efficient than one?

I have two questions:

1. 
   

   Is two calls to std::cout less efficient than one?

   
   


2. 
   

   If yes, does the compiler (usually) optimize it and generate equal assembly for the three cases shown below?

   
   

For example:

std::cout << "a" << "b";

or

std::cout << "ab";

or

std::cout << "a";
std::cout << "b";

I ask, because I think the last one is the most readable for a longer list of text to output.