Create new column if first letter is a specific letter in R

Consider the following data in R:

d <- data.frame(a = c("E5","E5","E5","E5"),
                b = c("011","012","013","111"))

I want to add a new column that is equal to "A5" if the first letter in column b is 0 excerpt "013". That is, I want the following table:

    a   b    c  
1   E5  011  A5
2   E5  012  A5
3   E5  013
4   E5  111  

How do I do that in R?